Because it can be super relaxing, as it was claimed in case of Weightless by Marconi Union. There are of course also so called meditation music and soundscapes, like sounds of nature, Tibetan singing bowls and binaural beats.



Marconi Union - Weightless Part 5 from JustMusicTV on YouTube

And if that is not long enough, you can also try the 10 hour version of it.

But music in general can of course stimulate or put the mind in rest. And what works for human mind can work even for plants. So, I very well understand if someone wishes to listen heavy metal or classical music while creating. I actually do like somewhat all kinds of music as long as it is good. Maybe the best music is what does not try to fit into any genre. Or stretch Metallica's Nothing Else Matters 800% slower to make it sound like ambient music.