Dill is the daredevil who bets Jem that he won't have the courage to go up and touch the Radley house. (This triggers the incident where Jem loses his pants.) Dill also brags about the father he doesn't even have. He has learned to tell "whoppers" and get away with it, so he "spins yarns" by the yard. Later in the story, it is he who gets the idea to go down to the courthouse to see where Boo had been chained up before his father brought him home. Dill puts a little spice in the Finch children's lives by inciting them to take risks and enjoy the thrill it brings. A bit naughty, Dill does not "mind" his aunt as well as Jem and Scout do their father. He defies adult authority in a way that the Finch children would never dare.
Note that Dill comes from somewhere else and he has experienced other things. He has even seen "Dracula!" His very presence helps Jem and Scout "grow" by extending their horizons beyond the little town of Maycomb and life with Atticus and Calpurnia.