The big reason to make jokes is because they're the best way to get a quick read on the collective mind of the group you're talking to. The volume of the laugh is important, but so is the lag time. You can tell immediately if they're on your side, and if they're really following what you're saying, by how quickly the crowd responds to your jokes.
Steven Berlin Johnson, on how to judge the silent tone of a room when speaking in public. See also his comments, with mention of the fundamental role of call-and-response in African American public speaking (and performance) culture.