They are to rank the comments and give the users karma points. So yes they have a purpose.

OP points out a real issue, though. If you didn't already know that, how would you? Perhaps some alt text could be added to the arrows.

You make a great point. It probably took me a day to figure out what "karma" was, upvotes, etc. I don't think I sent a single upvote for weeks since I didn't know how important they were/weren't.

HN isn't really set up for the new user, IMHO.

I'm pretty new here (about a week I think). While the voting/karma system isn't readily apparent, like you, I figured it out about a day after registering. I think the FAQ could possibly benefit from a more thorough explanation, but I do think people figure it out eventually once they start participating.

I registered yesterday and came to know about it immediately about what he up arrows are. Anyone who's familiar with discussion forums can get it in a while I guess. But, sure, I think HN should have given a note of this. Using Alt text is a good idea to keep the space clean and unintuitive.

I agree. If I had never used anything like Reddit or Digg, I probably wouldn't know what the arrows meant.

getting rid of karma points on comments has certainly hurt here... it's harder to ascertain what they are for when they don't increase.