As I see it, the two most important things about repeats are so absurdly obvious that their implications can be easily overlooked. The plain but pregnant facts are, first, that a repeat allows a piece to be heard twice; and second, that it makes the music twice as long.
Hearing the music twice is an advantage if the piece is complex, subtle, original, profound and at the same time terse.
John Gibbons, liner notes, The Goldberg variations of Johann Sebastian Bach.
A terse writer, I copied out this passage years ago because I loved the string of adjectives preceding it.