For me it is different reason at times. Most often it is to numb what I am feeling emotional pain. The sense of relief provides a feeling of being in control of myself and like I have the ability to cope.

Other times it is to express anger at myself. When I get filled with self hate, it gives me a way to express it and let it out. Afterwards I feel calm.

Other times it is when I am disassociating and I want to snap back. I feel really numb, lifeless and not there to the point where it is disorientating and I'm not sure anything is real. Cutting when I am like that snaps me back and I start to feel things again.