1) Exploration of new music.
2) Annual music camp for high school students -- we don't know until a couple of weeks beforehand the final participants, and IMSLP has been invaluable for finding unusual combinations, or collecting works without violas (always in short supply!), or works where I can use Eb horn parts for alto sax players, or just for downloading and seeing if a work is too easy/too challenging -- things you can't do if you don't have time to order music, or need to look at it before buying.
3) Orchestral parts for a university Orchestral Studies course -- the students download their own library of auditon works.
4) On one occasion one of the university oboists forgot her music at a rehearsal, and she just went to the library and printed off another copy -- simple!!!
[edited to add] 5) A lot of autograph manuscripts are now on IMSLP, and they are invaluable for research, and also give access to music not available from the publishing industry.