The College Football Playoff started in 2014. It produces its own weekly rankings beginning in the middle of the season and they determine the two teams that will play Alabama and Ohio State for the national title. I kid, but almost every iteration has involved Ohio State, Alabama, Clemson, and Oklahoma. There are basically two rules: 1. Alabama is in no matter what, and 2. No non-power 5 teams.
The rest of the top 25 is an honor, and usually sued as a measure to justify rule No. 2 by inflating other team’s profiles. Now, for the first time ever Purdue is in the top 25.