In the past, they'd have to make cuts and some guys would get very few chances to show what they could do in just the one preseason game prior to first cuts, esp on teams with a new QB where he'd play with the first unit for the first half or on a D installing a new scheme where the starters needed significant snaps.
This way, everyone will get snaps, and gives the coaches more game chances with the entire roster to really pick the best guys. Also means if a guy has a soft tissue injury in TC, he's not an automatic cut as he’ll have two more games to show out.
It removes an artificial barrier and increases eval time. It also saves time for coaches/FOs because they will only have one cutdown day.
Lastly, it means only one day of anxiety for players instead of the reality show format it was.