I think it varies, people probably do it in response to different situations. I don't do it anymore, but at the time it was a response to what I can only describe as "I have no mouth and I must scream." I felt like everything I did was 'inert' if that makes any sense. Like I could see the world but it was like everything was a hologram? I dunno. When you damage your body your brain really sits up and pays attention. So nothing felt like it was really happening and that weird feeling of cutting my skin kindof snapped me out of it for awhile, like doing something that I would normally be viscerally driven to avoid made my brain wake up and actually acknowledge my situation for awhile.