The disc doesn't have to have the full data on it..in fact most don't nowadays and they require downloading the additional data from the store...so there shouldn't be any issue here in terms of storage... essentially what a Switch 2 game key card is.
It's for a physical license and entitlement of ownership
This is Sony wrestling away the last bits physical ownership they can take away to force the middleman out and make used games history. This is going to result in higher prices and significantly fewer sales
Having the full game isn't possible anymore for most AAA games This is the only compromise...and they're taking that away now too