Lee uses it to reinforce the hypocrisy inherent in Maycomb. While the blacks worship there on Sundays, the whites use it for gambling during the week. This subtly reinforces the double standard that exists in Maycomb for blacks and whites.
It also serves to reinforce the dual nature of the main characters. Previously, we have seen that Atticus is not as old and boring as Scout thinks when he shoots Tim Johnson. Likewise, we have seen that Mrs. Dubose is not as menacing and evil as Scout and Jem thought either. Also, we have seen how the children think of Boo as this neighborhood legend who dines on cats at night. However, another side of Boo will be revealed.
In this chapter we learn that Cal has a different side to her. She speaks differently around her fellow African Americans than she does 'at home' with Scout and Jem. Scout notices that Cal lives in two different worlds.
The reader can also see that Scout is living in two different worlds too - one of innocence while she is about to enter one of experience (thanks to the looming trial and its effects), one of being a tom boy while she is about to enter one of womanhood (thanks to Aunt Alexandra moving in to try and exert some feminine influence over Scout), and one of youth (playing childish games with Dill) while she is about to enter one of adulthood (learning about Maycomb's caste system and how she is to fit into it).
Jem and Scout's visit to First Purchase African M.E. Church is significant for many reasons. First, the children experience racism for the first time as white visitors to a black church. When Lula insults Calpurnia and starts up the pathway toward the children, Jem says, "Let's go home, Cal, they don't want us here--" and Scout agrees. However, there are many more people, including Reverend Sykes, who are glad to have the children visit.
The visit also exposes Jem and Scout to several crucial attributes about Calpurnia's church and the black community at large. First, they take care of their own: a collection is taken up for Tom Robinson's wife, and everyone is pressured to donate. Secondly, they learn how the churchgoers overcome their illiteracy by "lining" the hymns rather than reading them.
Perhaps most importantly, Jem and Scout see Calpurnia in a new light. They realize that she leads a "modest double life," that she is highly educated and respected among her peers, and that she will defend them against anyone if she needs to.