Depends whether you're talking intended purpose or actual function. The intended purpose of the karma system is to make low-quality comments less visible. It doesn't do a very good job of that here; it does a better job in Reddit where there's a larger userbase and threads are sorted by karma by default, since most people don't read all the way to the bottom of a thread, but collapsing threads doesn't do much.

The actual function of karma is to gently incentivize posting things interesting to the community, to somewhat less gently disincentivize content-free posts, and to not-at-all-gently dissuade persistent cranks and trolls and short-circuit discussions of things that're really strongly disapproved of (the so-called troll toll, though most of its victims are not trolls). I don't think this is a bad thing on balance -- community can be undervalued in nerdy circles, so it's handy to have a semi-mechanical way of encouraging it without everything degenerating into cat pictures -- but one shouldn't mistake it for something it's not.

I haven't seen much evidence of widespread use of sockpuppets.