The same point everywhere - to drive discussion to popular posts and away from unpopular posts, and to encourage conformity to board culture through operant conditioning and gamification. If you give people a number and tell them that number is special, they'll do whatever they can to make that number go up, and to avoid whatever makes it go down.

At least in theory. In practice it's utterly useless because it's based on incorrect (or possibly outdated) assumptions about the nature and goals of HN's userbase (which I've decided to call the "good hacker" fallacy.)