The title says it all. I often heard people say something like memory is unimportant in doing mathematics. However, when I tried to solve mathematical problems, I often used known theorems whose proofs I forgot.
EDIT Some of you may think that using theorems whose proofs one has forgotten does not seem to support importance of memory. My point is that it is not only useful, but often necessary to remember theorems(not their proofs) to solve mathematical problems. For example, you can't solve many problems of finite groups without using Sylow's theorem.