1) If you are a P5 conference - To keep the G5 and non-ND independents out of the CFP, keep all the money to yourselves, and lie to the world that you are the next best thing since sliced bread.
2) If you are a G5 conference or independent - To get a fair shot of finding out who the champion is.
Who runs the CFP? Answer that question and you'll know which answer above is accurate.