I think most of the time it's both meaningless and meaningful. With me, I feel like I have to tap things for different reasons, but mostly to distract myself from thoughts. Like, I have a feeling that my brain's about to flood with terrible thoughts, and I go and tap something to distract/prevent it, or when I have a thought I go tap something to "get it out." But it's a totally meaningless action if I think about it. Tapping on a cabinet or a desk or a door or whatever, what is that? And it doesn't even really work. The thoughts come in as they please no matter what I do, but I have this feeling that if I didn't do compulsions things would get worse.. but the compulsions are really annoying but I still feel like I have to do these things. Ugh OCD is so weird.