They higher rated team is helped by beating a strong team, it enhances their resume with the committee. Ohio State last year is a great example; in danger of being left out of the playoff, they soundly beat a top rated Wisconsin team which cemented their slot in the playoff. If they would have played a non-rated team, there is a good chance tOSU doesn't get in the playoff.

The second reason is if the higher seed loses. If you have a #1 or #2 ranked team that loses, but they are beat by a #5 or #6 ranked team, there is a good change the winner of the CCG still goes to the playoff.