I do it first for the sake of creativity, then, for experience, if I ever get "THE JOB" which doesn't seems so imposible, at least at a lower scale... then, to have my "repertory bag", for any use, hahahaha. And last, because I write the music I'd like to hear for the moment
Even tough I could never performe, I guess that wouldn't be at all bad. But I do always worry about any details in the interpretation issues, and actually, I'm pretty personalized with each part of "my ensemble"