I've always wondered about them myself and have only reluctantly included them in courses. They've always just seemed annoying to me. Well, I recently took Will Thalheimer's Original Learning Research Quiz which had me rethinking the whole point of prequestions/pretesting.
Will points out the validity of pretests and prequestions, arguing that
"prequestions function as learning objectives" guiding the user's "attention toward some aspects of the learning material and away from other
aspects of the learning material."
So if pretests/prequestions are a way to focus your user's attention on the important aspects of the content, then the big question here is, are you asking the right questions? Or are you just asking questions for the sake of asking a question -- because someone told you to include a pre-test?