turkey with beautiful browned skin that doesn't stick to the meat
When it's in a bag, the steam produced during roasting is trapped.  A layer of steam occurs between the skin and the meat, separating the skin from the meat.

The same process happens when cooking without a bag, just more slowly, as the steam fills the entire oven - and then some of it escapes through vents or out the door seal.  

Turkeys release a surprisingly large amount of liquid when cooked, compared to other poultry, so the skin separation is more pronounced than in chickens.