Think of it like an arbitrary, minor skinner box that's sometimes noticed but rarely talked about (because few people pay it any mind), created solely to draw you in further once you're already here, and have you post more topics or replies than if you had just visited one time. Does it necessarily make sense? I'm not so sure about that. It's almost doubtful that SE gets much out of such a system if that were the case, trying to get more people to post more often, unless they really like all of the salt around here.