I agree with the above-noted reasons for this ACC/B1G/PAC-12 alliance, but two other things about it stand out to me:
1) They didn't bother including the Big XII minus IV, or Big 8, or "the Left Behind," whatever you want to call them. I think this cements that league as a tweener at best, and more along the lines of the American & Mountain West.
2) This alliance is probably also geared towards influencing/revising the format of the expanded CFB, putting a cap on the number of bids any given conference can get per season, and trying to make sure that the expanded playoff isn't just handed to ESPN but goes out of the open market for bidding (maybe to two different networks, like the AFC/NFC playoffs being on different networks or March Madness now being on both CBS and Turner networks). Not sure if they want to drop down to 5 conference champs or keep it at 6 as proposed, but the potential for multiple SEC teams in a 12 team playoff has to piss them off, even before OU and Texas join. Using last seasons final pre-bowl CFP rankings, Bama, Clemson, OSU, OU, Cincy and Coastal would've gotten autobids as the 6 highest ranked conference champs, and ND, A&M, Florida, Georgia, Iowa State and Indiana would've gotten at large bids (so 4 SEC, 2 ACC (counting ND cause of weird COVID year), 2 B1G, 2 Big XII, 1 AAC, 1 SunBelt). I think other leagues are going to want to limit at-large, non-champion bids to 2 per conference, or 3 total, so that no more than 1/4 of the 12 team CFB playoff can qualify. That seems dumb to me, particularly when the PAC-12 champ wouldn't have made it anyway, but the SEC is the bigger 800 lb gorilla and along with the Mouse the other leagues are going to do whatever they can not to get steamrolled.