Weird addiction pretty much sums it up in my case. I've been cutting since I was eleven-ish years old and I still can't quite explain it. I think part of it is just to direct all my emotions to one spot, kind of like how some people punch walls in fits of anger. When I'm overwhelmed, I need to take it out somewhere, and when I'm numb, I just want to feel anything.
Honestly, I think part of it is just to have some physical evidence of my pain. Like, "look, I really am in pain." It's not just in my head. It makes me feel like I have an actual reason to feel like shit when there doesn't seem to be a good enough reason for my problems, like I'm validating my own mental illness and depression in some bizarre way. It's a way for me to simultaneously wallow in self pity and punish myself for wallowing.
I don't think any of it is "worth it". It's not like it's done me any favors in the long run, just scars to hide and uncomfortable questions and strangers who stare when they catch a glimpse. There's a lot of embarrassment of fitting the stereotype of a dramatic emo teenager looking for attention, too. But it makes me feel like I'm accomplishing something, somehow, and pain is an old friend. And at the end of the day, it's an addiction. I've tried to stop a couple of times and I never managed to. I literally nicked a box cutter from work just a few weeks ago because I had the opportunity. It's second nature at this point.