The purpose of proofreading is simply to catch any errors that were missed on previous passes and eliminate any new ones that may have been introduced between line editing and layout. (editorialdepartment.com)
You can also go back to previous passed levels with new characters and find new unlocks, which gives the game real longevity. (macgamerhq.com)
In previous passes through the plumes, the Cassini team found organic molecules like methane and formaldehyde. (sciencemag.org)