Schools are able to shed players who either don't want to be here, or aren't getting enough playing time. That opens the door to fill "positions of need."

Completely hypothetical situation: lets say a school completely whiffed at a position like D-line for a couple of recruiting cycles/years. The "portal" gives that school a chance to pull in a few grad transfers to cover the recruiting gap. If you need two years covered, you hit the portal in back-to-back years to fill your position of need.