I think sometimes it can be an avoidance thing for me. Like, it makes me focus on the SI behavior, or the pain of the SI'ing instead of what is really causing the problem. Usually this is also tied in with other reasons though.

I have SI'ed as a distraction technique/aviodance issue with other people though. I won't want to talk to them about what really is the problem, so I let them "discover" cuts, (I only do this with people who already know about the SI'ing) and then they focus on that instead of other problems. Not to be confused with SI'ing for attention, as I usually hide my cuts at all costs, and this is the last option I'll use when someone is getting too close to issues I don't want to talk about. Cause it's easier to talk to them about SI than the real issues.