When first time I played NieR, I thought the way the BGM varied in intensity was super compelling, but I couldn't really articulate why or how until I looked at the original soundtrack (OST). If you look at the pinned comment with tracklist for the OST, you'll see that almost every song has several versions. This is similar to the instrumental, hummed and fully voiced versions of songs from, the indie game, Transistor for those familiar. For NieR, they have Quiet, Medium and Dynamic instrumental tracks, and on top of that, they have those versions with our without vocals. Each version varies in intensity, generally adding audio tracks in each version, but also with some tuning to remove the more ambient sounds in Quiet for more aggressive and distinct instruments in Dynamic.
There are three major interwoven strengths that come from this decision I'd like to spend some time discussing, no story spoilers, though I will talk about two zones in the game. 1) Flexibility. 2) Pacing. 3) Coherency.
To begin, it's important to note while having flexibility by having variations of songs is not rare, it is also not the norm. Many games use single themes for entire zones (every game in Final Fantasy), and then employ other tactics to keep the world coherent by using leitmotifs or creating different versions of songs with that shares melodies. The key difference between NieR and these other games is that they create new variations of old songs for coherency, while NieR absolutely does it for flexibility.
I want to say that the topic of "flexibility" will come up again over and over again in the following sections so I will keep its discussion brief with two examples. Following with the Final Fantasy franchise, they do this all the time, iterating motifs from its main themes across different songs. Another notorious example is Undertale, where in addition to countless variations of several prominent melodies for every mood of the week, fans have been able to pick up many of the subtleties in the storytelling in the game's music. Finding out how "X character's theme is actually Y theme at 50% speed, or how even a villain's theme may have this type of relationship with another song," these intentional relations reinforce the twists and surprises in the story, even priming the player ahead of time of whats to come. The argument I'm making with NieR, is that while many sound directors will smartly use leitmotifs and variations of songs, NieR's song variations are much more extensive and for different reasons. The flexibility of NieR's soundtrack aids in its pacing compared to other games and actually adds to its coherency in a different way that other games ignore.
There are generally two different ways the flexibility of Nier's theme variations operate. A) They either are used successively to add tension as the story progresses through different areas within a zone, or B) they are used to highlight specific moments. Often it's a little of both.
There are two zones in NieR I'lll use for examples. The Amusement Park and the Desert. The Amusement Park generally uses its variations for pacing your proximity to the punchline. The quiet version of the song is ominous, you are on the outskirts, you don't know what to expect, you haven't seen what's up ahead yet. Medium plays, the tone of the zone is made clear. As my roommate put it, "the song is dramatic but…" It has strong contradicting elements, a playfulness with its chimes but also a sinister underlying melody, you see the fireworks in the distance, the balloons, what appears to be an Amusement Park. Dynamic + Vocals plays, the heavy drumline enters and the vocals are no longer hidden. You're in the midst of it, you're up close, seeing the details and everything that confirms your suspicions when you first heard the Quiet version.
Very important to mention is that in addition to adding tension, there is immense flexibility in being able to use softer variations to cool down, which is extremely important in RPGs. For the back-alleys in towns, in hidden passageways, in every zone, it can even be argued that artful deescalating pacing is more important than the build up. A boss can enter the area with a surprise, the boss theme plays, we beat the boss, a cutscene plays and the next scene we're back somewhere else, perhaps in town listening to upbeat town music. Zone Music –> Boss Theme –> Cutscene –> Zone Music 2. The deescalation is fragmented with different songs that aren't related. In the Amusement Park, it goes something like this: Quiet –> Medium –> Dynamic –> Boss Theme –> Medium.
While every region in NieR uses the variations for general pacing like this, there are moments when the variations are used to highlight specific moments, and this will be a short and good transition to show how there is a coherency in NieR that other amazing sound directors don't go for. The Desert theme, has its variations, but we don't hear the Dynamic version for a long time. Instead we hear Quiet, Medium, and Medium + Vocals. The Vocals are already very powerful, so as we explore the expanse of sand, the world is engaging as it is. I won't spoil what you find in the Desert, but an extremely key difference between the Medium and Dynamic versions is the addition to a large bass drum at the beginning of the Dynamic version. This beat lands right when Dynamic version starts, right when you see in front of you this revelation. They create a world of sand with this long stretching song, but to contrast this moment from the rest of the zone, they reserve this variation of the song to make sure this scene has impact.
Finally, so what is it about coherency that I'm arguing NieR has that other games don't? Consider Skellige of Witcher III, whether you're on the highest mountaintops, by the coast or in lowland woods, Ard Skellig's soothing and huanting melody echoes. The moment you step foot on the continent you feel at home, you feel familiar with it. You can imagine so vividly what it would be like to stand there on the wild fields, close your eyes under the sun with the cold wind blowing. When the theme is tuned just right for the environment, shouldn't its flexibility already lead to the ultimate consistency? I argue that Skellige is extremely immersive, but more than half the props goes to the Art team. How the grass sways, how the river runs, the light and shadows play, NieR doesn't come close, doesn't even attempt to challenge that front. Witcher is a damn immersive game, but NieR still has a one-up, and that's how cleverly they use their soundtrack.
It's the way NieR fades and mixes these soundtracks which give them a coherency that other games don't have. A great example of this is how amazingly they switch to the 8bit version of songs for hacking sequences. While the hacking minigame can lose its charm after some time, I'm sure for anyone who first played NieR, when you get hacked in that particular fight, you must've noticed how the song shift shifted to the chiptune version. I argue that isn't just something they did for fun that the fans might like when the sound team had some spare time. I think think that decision was the single thing that made the hacking sequences playable. What could've otherwise been an annoying, disengaging minigame that throws you out of the combat and throws off your timing is made palatable by the sound team.
Another example of coherency in NieR is in the menu. In other games when you go to the menu, they lower the BGM. It feels like you took a pause, but in NieR, they fade to the different version (usually instrumental version if I recall right). It still feels like a pause, but it keeps you more engaged, when you resume the game, the vocals kick right back in. When you go from desert to the moment in the desert with the revelation, the song changes and starts on the first note with the impact. When you go from the alleyways of the Amusement Park to the mains treet, it shifts versions. It's how all of these the BGM switches from variation to variation that they keep entire zones coherent. Imagine if Ard Skellig had a more austere and frigid version when you were in the coldest regions or a more subdued version for nighttime (NieR doesn't really have a day-night cycle like Witcher)? This is what I mean by how an entire zone is made more coherent by using different variations for the zone's theme.
Of course there are pros and cons. One thing about Final Fantasy is that heck, even if the zone's theme plays on loop for the entirety of the dungeon, it's distinct, it's memorable. A lot of people critisize the soundtracks for anime, movies or games that overly rely on generic background or "ambient" music. They really don't get in the way and let the rest of the medium do the talking. I'm one of those people that really enjoy BGM with strong melodies that take a strong forefront. NieR uses a lot of ambient music, which is, by nature, extremely forgetable, but it also has variations of these Quiet tracks with full body and are unforgettable. I'd say it's the best of both worlds.