I believe that laughter exists for the purpose of combating negativity borne out of consciousness and the human need to "feel good" in order to succeed. However, it could be for other reasons like its power in social bonding, piggybacking onto the development of intelligence, or even a neurological misfiring.
I think humor is like The Tao: once you've explained it, you've explained it away. You can say "it's funny because it's true", and be right; but you can also say "it's funny because it's not true", and be just as right. Humor doesn't seem to make any sense outside of the facts that what is funny and how it is created is relative to intelligence and culture, and that there don't really even appear to be any rules and it can permeate just about anything
Part of me wants to say that laughter exists because life's a bitch and then you die, and I guess if you can laugh about it, you do a little better than people who can't. Another part of me wants to say laughter exists because it's excellent for social bonding