Soundtracks are both the most useful and the most enjoyable music out there.
In real life, there isn’t a constant backing track perfectly matching everything we experience. But in visual media, there’s no reason that silence can’t be filled. And while we’re at it, why not make it do something useful? No matter how great the acting, there’ll always be some ambiguity as to what a situation feels like for its characters, how they experience what’s happening.
A soundtrack is a great way of getting closer to that unreachable 100% of clarity. Sure, your viewers will still interpret the backing track individually, but in combination with good acting it helps narrow the different emotions someone’s likely to feel down to a smaller group. And that’s super useful if you want to make sure your work gets across in just the right way!
Of course they can be used for ambience too. Especially in games, where your surroundings are usually more important than your character. It can make the difference between a dark cave full of wonders to explore and a grim endless maze of no return. If there’s desperation, make your players hear it, drum it through their ears, into their hearts.
Listening to the soundtrack of the latest Witcher game, it’s amazing!