I've implemented a couple of user types. I never really understood what the method returnedClass() is for; why does hibernate need this?

(Un)surprisingly, I found that this method is never called while saving/retrieving objects. nullSafeGet() and nullSafeSet() are called, but returnedClass() is never called.
Simply returning null from this method doesn't seem to cause any issues.

Any idea what this method is for? Is it necessary to provide proper implementation of this method?