Much of the braising guidance I've read emphasizes the importance of a tight-fitting lid. For example, in All About Braising, pp. 18-19, Molly Stevens provides instructions for sealing a braising pot with parchment paper. The goal is to reinforce the seal since "much of the success of braising depends on trapping moisture in the pot."
On the other hand, Harold McGee, in his "Guidelines for Succulent Braises and Stews," advises braising with "the pot lid ajar to allow some evaporation" (On Food and Cooking, p. 163). J. Kenji López-Alt, in his Food Lab article on Great Chile Verde Without Hatch Chiles, notes that a pot with its lid left slightly ajar "stays a good 20°F lower, keeping the meat inside at a temperature far closer to the ideal."
How do I make sense of this seemingly contradictory advice?