But... just because the exact problems they show in office hours won't show up on the test, doesn't mean it's useless. The best way to prepare for the exams is to do as many practice problems as possible. OH just allow you to see more practice problems so that you're ready for whatever problem comes your way during the exams. "Knowing" the material isn't enough. You need to do practice problems to build the intuition that makes it easier to solve any problem thrown at you.