I often need to start some work with music, because I constantly have an internal jukebox running. A lot of people listen to music to fuel that narrative illusion, which gives them motivation, and some do so to block out more distracting noise, as conchis mentioned. Mine is a variant on the latter, so I just get something with low information content stuck in my head before sitting down for serious work.

(I make an exception in certain stages of writing fiction, since I often soundtrack scenes as I create them, or even create scenes that fit the melody.)