I don't have any experience of working in film, so all I can do is try to understand this through my own frame of reference. In this case, that is mixing a piece of music. I know kleinbl00 has plenty of experience there too, so maybe he can confirm or deny my understanding.

I'm far from a master mixing engineer, but what I've learnt over the last few years is that the mixdown process is greater than the sum or its parts. It hundreds of little changes that, in isolation, don't seem to make that much difference.

For example, you could:

- Add a compressor to make sure the volume of a vocal performance stays consistent

- Add some subtle timed delay to a snare drum to give it a greater sense of power

- Setup sidechain compression between the kick and the bass so that they're not fighting for space in the mix

As a composer, it's very easy to look at those things and think: "none of that stuff is stuff I care about". I care about the arrangement, melodic themes, and chord progressions. They're the big, important things. What difference is a bit of EQ going to make? If the listener is focusing on the vocal compression then there's something more seriously wrong with the music.

But if at the end of the mixdown process you reverse all those tiny adjustments, suddenly you're greeted with a significantly more confusing and harder to understand piece of music. Yes, a lot of the changes can seem almost subliminal and inconsequential to the average ear. But add enough of those subtleties up and suddenly it's not so subtle. And ignoring them all results in less comprehensible and immersive experience for the audience. That's why there's a professional their who can sense those things making those changes.