For decades, acappella recording engineers have double-tracked the singers who perform the backing parts in an arrangement. All the tenors, altos, sopranos, harmonies, auxes, adds, trios, and even the basses end up with two (or sometimes more) separate and unique tracks of their entire part. Why? Danny answers that question with this video.
BONUS: he even shows you how the pros handle a divisi/split in a part when recording.