atime is heavily used by traditional mail clients like mutt, to determine which mailboxes have new mail: If atime < mtime, then there has been a write to a file since the last read.

Because updating atime on every file access carries an IO overhead, there's a mount option "relatime" that only updates atime if atime < mtime. This preserves the "new mail" feature of mutt, but doesn't require atime writes for every file read operation, which is a decent compromise.