For Scout and Jem, summer means Dill, and Dill's imagination: "Thus we came to know Dill as a pocket Merlin, whose head teemed with eccentric plans, strange longings, and quaint fancies" (1.39). The Finch kids may have active imaginations, but they're firmly entrenched in the reality of Maycomb. Thanks to Dill's outsider status, he can see the Maycomb community from a different perspective.
Take Tom Robinson's trial. While Scout accepts Mr. Gilmer's rude treatment of Tom on the witness stand as normal, Dill starts crying uncontrollably when he sees Tom being treated so differently from the white witnesses. He can't quite explain his feelings, but Mr. Raymond can.
"Things haven't caught up with that one's instinct yet. Let him get a little older and he won't get sick and cry […] about the simple hell people give other people—without even thinking. Cry about the hell white people give colored folks, without even stopping to think that they're people, too." (20.20-22)
Dill's sensitivity to Maycomb's intolerance gives Scout (and us) a different model of how to respond to what's happening. The contrast between Dill's angry tears and Scout's justification of Mr. Gilmer's attitude with the surprisingly callous "he's just a Negro" (19.164) suggests that Scout's already been hit with Maycomb's ugly racism stick. Not even being Atticus's daughter has been enough to shield her entirely from her community's prejudices.
While Scout and Jem struggle after the trial to make sense of the Maycomb community that they thought they knew so well, and to figure out their own place in it, Dill takes a more detached approach:
"I think I'll be a clown when I get grown […] There ain't one thing in this world I can do about folks except laugh, so I'm gonna join the circus and laugh my head off."
"You got it backwards, Dill," said Jem. "Clowns are sad, it's folks that laugh at them."
"Well I'm gonna be a new kind of clown. I'm gonna stand in the middle of the ring and laugh at the folks." (22.60-64)
While Mr. Raymond predicts that Dill will grow out of crying into not caring, Dill himself comes up with a different path, hiding the tears in laughter. Both responses, however, are difficult for Scout to understand. Dill's character suggests what the limitations of Scout's perspective might be, giving the reader a broader picture of what's the matter with Maycomb through the different limitations of Dill's viewpoint.
Famous American writer Truman Capote was a childhood friend of Harper Lee's, and he insisted that he was the model for Dill. Capote didn't grow up to be a clown, but he did grow up to be an insightful—and sometimes harsh—critic of American culture. Maybe Dill won't grow up to laugh at the world, but we think he'll grow up with something to say about it.