The Free Software/Culture stuff is also about DRY, in a sense. Say I need a picture of a stream for a project. I get my camera and drive a couple hours to the woods. Batch process the set and finally decide on one picture to use. At the end of this, fairly expensive process, I actually have 10-15 pretty good, usable pictures that I no longer am going to use (at least not in the near future). The (modern) traditional ideology is to lock those away just in case I may need a picture like that in the future.

Makeing them free to others means they don't have to repeat the same process for basically the exact same thing. Now multiply this by the 100's or thousand's of other assets that could be used in a small project.