1.) Laughing at something that's not even funny so you don't seem rude.
2.) Laughing in order to avoid awkwardness.
Person A: Did you see that music video
on Youtube about Humpty Dumpty? What a CRACKhead!
Person B: Haha. (Conversational chuckling)
Person A: Dude, look at that girl's sunburn!
Person B: Hahahahahahahaha! (Conversational chuckling)