As an amateur pianist and organist, I have piles of old sheet music, donated by family and friends, resulting from attic clearances, car boot sales and whatever. Among them, quite some unknown or forgotten music. Although probably forgotten for some good reason, I find it a pity to just throw these away -- for ever. After all, some fellow musician took the effort (and even enjoyed) to compose it, somebody did typeset it (without Finale or Sibelius & Co!), an entire sales apparatus took care of the distribution, and at least one fellow musician bought it and hopefully enjoyed it.
Fortunately the IMSLP project allows me to add my little step to this chain, and save them from oblivion. Before I finally get rid of them anyway (my attic also needs clearance from time to time...).