Some RPGs have SRDs. Wikipedia tells me that they are System Reference Documents, but that it's largely a D&D thing, and doesn't explain why they exist. Only that they do.
Even if that were not the case, from the ones I've read, you could largely figure out how to play from SRDs alone, so why do they exist? Is it actually more beneficial from the creator's point of view for them to exist over any losses from sales? (And, I guess, what goes in an SRD and what does not?)