Dudes needed a bit extra time to get ready, and were part-way through the season. They can now step in and play when they're ready, instead of sitting out. Then they get to come into the next year as a RS freshman with PT under their belt. Pretty awesome.
It's nice for programs in general, with freshmen and guys who hadn't been ready or were maybe injured. But BYU gets the extra benefit because instead of just having true freshmen right out of HS, there are always some RMs who might be close but not quite there early on, and then ready to step in and get great experience later on.