I read an article explaining that if players are recovering from injury they will sometimes "Fail" their physical so they can be placed on PUP. This allows us to keep them without taking up a roster spot during camp. It buys some time to look at younger guys in camp, while adding a safety factor to the PUP player's recovery and ensuring they're good to go. Its important to note that as soon as a player is cleared medically, meaning they pass their physical, the organization becomes immediately responsible for them financially... so... you dont want a guy aggravating something, then swallowing a roster spot and a salary when he cant play
Its was in Vic Ketchman's discussion on Packers.com this week. I probably butchered his explanation but hopefully you get the point lol