It's just an easy, anonymous and efficient way agree/like or disagree/dislike, nothing more. No one is awarded any points, no one loses points, downvotes don't mean it'll be removed and upvotes don't automatically guarantee it'll be MH either. Hiding them until you vote has drastically cut down on the "trend voting" and people seem to put more thought into whether they upvote or downvote... it's been interesting to see that change.
It is a way to quickly and anonymously tell an opinion author that you liked or disliked what they wrote. Unlike adding an anonymous reply that says "I like what you said" and "I dislike what you said", the author is not given a notification that a reply was made. That's about all there is to it.
It's just a rating for your answer. Some users may be interested in how others feel about their answer. Did others like it? Dislike it? What could be the reason? How can I make better answers in the future?