Specifically with the CGs it is a single pinup full color illustrated image that will give life to a specific ending. Because each route has a different combination of weight distribution, the CG would exemplify what that route was about giving it a nice capstone reward.
Other tier rewards include a new location, making the game 4 times larger, and an extra “week” in the game play which makes it an additional 33% larger after that!
The final tier is a bonus ending that can happen in any route (after certain conditions) that give a completely different ending and ending image.
Thanks for the question I hope this cleared it up!