As I understand it, a good part of my job as a composer — my discipline, if you like — is paying attention to music other than my own, listening to it, playing it, finding out something about what makes it work (or not, as the case may (also usefully) be). I like to work with scores, to read through them, play through them, and — here is my own old fashioned (& maybe slightly perverse) way or working — copying or transcribing all or parts of them.
There is something deeply satisfying about being so close to a piece of music that you can rightly claim to have spent time with every single note, and not just listening to it or playing it, but actually writing it down in its scored context. I used to do this by completely by hand (& I swear there's no better training than copying by hand) but now I enter it into a notation program, which speeds some elements of notation up but I still insist on entering note-by-note, if only as a way of effectively slowing down my analytic listening process.* (I even use that program's playback capacity from time-to-time, but mostly for working with tempi or complex rhythms, a matter that I find enormously sensitive to the slightest changes; I don't ever quite trust the playback for its representation of pitches or instrumentation or articulation or balance, but it is one heck of a metronome. )
As part of my summer's plan, I've laid in my annual store of scores I'd like to know better and by the end of the summer, I expect that a good amount of them will have been copied, added to all the scores — from Machaut to La Monte Young — I've copied by hand over the years. It's an old habit, one that started as a teenager working in libraries, but not one I plan to give up any time soon.