Little details such as the practice of "lining" are windows into a culture. When Jem and Scout went to church with Calpurnia, they were surprised to find little of what they saw in their own church. In particular, there was no piano, organ, or hymn books. The children, therefore, wondered how people could sing. Here is a quote that shows what the children saw:
There was no sign of piano, organ, hymn-books, church programs—the familiar ecclesiastical impedimenta we saw every Sunday. It was dim inside, with a damp coolness slowly dispelled by the gathering congregation. At each seat was a cheap cardboard fan bearing a garish Garden of Gethsemane, courtesy Tyndal’s Hardware Co. (You-Name-It-We-Sell-It).
Jem and Scout asked Calpurnia how people sang, and Calpurnia told them to be patient. They would soon see. Here is the quote:
Zeebo cleared his throat and read in a voice like the rumble of distant artillery: “There’s a land beyond the river." Miraculously on pitch, a hundred voices sang out Zeebo’s words. The last syllable, held to a husky hum, was followed by Zeebo saying, “That we call the sweet forever.”
What this shows us is the state of the black church in Maycomb. The black church was not wealthy enough to have instruments, such as a piano or an organ. But they did have the most beautiful instrument, the human voice.
The lack of hymnals not only underlines their poverty, but also their inability to read. This is an important point to remember, because it is mentioned that Calpurnia was able to read. This detail shows that Calpunia was an exception. Finally, that the black church could still worship with so few resources shows us that people do not need much to worship - just a battered hymnal and willing voices.