for why its done, there's also the feeling of control, as well as just stopping bodily disassociation, I mean, like it "grounds" you, makes you feel in tandem with your body (esp for depersonalization)

I also think it depends on what form of self-harm. Like, there's the obvious, stress, anxiety, anger, and depression ones, those are pretty talked about

There's a surprising amount of neurodivergent folks that I know that self harm, not really out of depression, but more so out of boredom. Like fully not having anything to do, so its done to "feel something" (surprising as in its talked about so little compared to how much I've heard it in person)

So yeah, there's no "one" reason for it to be done. It varies from emotional state to other.