It has legitimate benefits. Aside from the ability to downvote comments after 500 karma, your karma score affects the positioning of your comments in threads. The highest comment or reply isn't always the most upvoted, there's a ratio of upvotes to cumulative karma that dictates where your comment sits in a thread and in turn, its visibility to other users.
It lets people show appreciation for a post even if they have nothing to say.
You get karma for every time someone upvotes you.
When you have 500 or so, you get the privilege of downvoting.
It's more of a curse than a privilege, I find myself hitting downvote now instead of replying with a well thought out answer to tell them why they are wrong.