What are the benefits of using a cooking bag to cook my bird?

I don't think there are any advantages. Some people love that method. Personally, I don't.

They're supposed to keep meat moist, especially the oft dried out turkey breast, by creating a steam cloud around the bird.

Personally I think they do nothing. Heat is what dries out meat, not lack of moisture. You can braise a cut to inedible stringy dryness with the meat sitting in liquid for the whole process. Far better to portion out the bird (spatchcock or quatering) and cook to temperature.

It helps keep the bird moist (my grandmother did this with aluminum foil, opening it just at the end for the skin to crisp up). That said, it isn't the only way to have a moist bird.