Announcing: a Leak!

Why does Wikileaks announce that they're going to release leaked material they have? Doesn't it at least kind of defeat the purpose of the leaks? Especially in the case of a private entity like "a big U.S. bank"? Doesn't it give those being leaked from time to fix the damage that might be caused? In the case of the government, if the leaks could actually put people in danger, this is understandable, but even then you wouldn't have to announce it publicly. I don't get it.