A great beer can add a new and unexpected depth of flavor, moisture, and gloss, especially when roasting meats.
I use it as a braising liquid as well. Be sure to use a good quality, flavorful beer and try to find one whose flavor will complement your other ingredients. But keep in mind that the flavor of beer intensifies in the cooking process; start with the lighter ones and then graduate. The caramel maltiness of a dark lager will lend sweetness to a roast chicken or turkey; the bitterness of India pale ale will be imparted to the beef and braising sauce; the wheat beers are mild and fruity and are good with fish. The quantity of beer added makes a difference: too much beer can make a dish unpleasantly bitter, so dilute it by adding stock or water, and start with small quantities then increase.