What's so funny about "dirty" words?

Last night I watched the Joan Rivers Roast on the Comedy Channel. All of the humor incorporated “foul language” to make people laugh by shocking instead of creative writing. Some of it was downright vulgar. I WAS embarrassed to watch it with my daughter. Carl Reiner’s routine was based upon saying dirty words. I’m not even convinced the people who laugh at the jokes realize that they’re not even funny, i.e., they laugh because of the shock (“Did you hear what he said?!!!). Or do they? Is there humor in obscenities?