-For self-punishment
-For relief of anxiety
-A release of anger (usually at myself so probably a part of self-punishment)

I used to do it partly for the self-care afterward too. It felt good to create a situation where I was harmed and then make it better. I guess an artificial replica of things going on in life? I don't know about you, but I have a hard time taking care of myself in general so for me and maybe for you it is a way to accomplish that.