The main purpose of the playoff is to make money, though they do it with one hand tied behind their backs because university presidents still wield a considerable amount of power, in partnership with their conferences, and they are, at least ostensibly, trying to feed the main mission of the universities - education - and look out for the interests of the student athletes.
That said, every method of crowning a champion is going to reward certain aspects over others. 64 team tournament (NCAAM), 7 game series (MLB), playoff brackets (NFL&NBA) all require slightly different characteristics to win it all. The goal of any playoff should be to reward the best team, but it turns out that's a more complicated question than it appears. No playoff system is perfect, it can't be, but what we have now in NCAAFB is better than anything we've ever had. It's still ridiculously inadequate. I'm ready for an 8 or 16 team playoff yesterday.