Because very rarely will players with a Alabama, LSU, Florida, Georgia, Tenn, Texas, Oklahoma or Auburn scholarship offer choose another school not in that group, so those teams will now collectively taking 160 players a year that would have otherwise gone to State, ole miss, Kentucky, Arkansas, etc. and been the best signees of the class. That's a lot of additional talent being kept with the top programs.
Plus it gives those teams 23.5% more room for error in evaluations, which is significant (though probably less so with the portal).
It also gives them more depth. Yes, it gives us more depth too, but our depth is now going to be guys that would have otherwise gone to Tulane, Memphis or ULL. Their depth will be guys like Bookie Watson, DeRunnya Wilson, Beniquez Brown, Jamar Chaney, Josh Robinson, etc. who were right on the fringe of getting offers from preferred schools but instead dropped to us.