Because often, people can take your work and make it far better than you ever could (yes that is a good thing) — or do new things in surprisingly interesting ways, like make a Office Space trailer that makes it look like a comedic thriller
(Yes I know that wasn’t creative commons based work. It might be fair use or parody or it might be illegal, but I seriously doubt Mike Judge is going to sue. The RIAA might, not because they have anything to do with the IP, but they like to do that anyway. That might sue me for saying that)