What is "linin", and why is it done?

"Linin" is when the song leader keads the congregation by singing them the words, which they then sing back. In To Kill a Mocingbird, Scout comments on the fact there aren't any hymn books, to which Cal responds it wouldn't matter anyway, as most of the congregation cannot read. "Linin" is the method Zeebo uses to lead them in song.

“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.”
Music again swelled around us; the last note lingered and Zeebo met it with the next line: “And we only reach that shore by faith’s decree.”
The congregation hesitated, Zeebo repeated the line carefully, and it was sung. At the chorus Zeebo closed the book, a signal for the congregation to proceed without his help.

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“He’s just like our preacher,” said Jem, “but why do you all sing hymns that way?”
“Linin‘?” she asked.
“Is that what it is?”
“Yeah, it’s called linin‘. They’ve done it that way as long as I can remember.”