Basting is “anointing” food that you are roasting with a liquid, usually the drippings off of the meat. It’s a relatively new cooking habit, not an old one: people just didn’t have ovens in which they could put roasting pans, which let the juices accumulate to be used for basting. Instead, they would have cooked meat on a spit over a fire, or boiled it so as not to lose any goodness of the meat into the fire.
Basting probably mostly came about because many people developed the habit, partly inspired by the Joy of Cooking, of roasting with water in the pan, and covering the meat with tin foil or the top of the roasting pan to keep the moisture in. Cooked like this, the meat, especially fowl, wouldn’t brown properly. The chicken and turkey skin would look white. Basting it with hot fat would help to brown it.
Basting does not help to keep the meat moister inside. The fat you baste a turkey with just runs right off, back into the pan.
That being said, it can be useful to baste in the last 15 minutes or so of cooking, especially with fowl, as it does make the skin more flavourful. If you are going to baste, take the roasting pan entirely out of the oven, and close the oven door to keep the heat in, do your basting, then replace in the oven. It’s also far safer this way, as there is no oven door to trip over.
The purpose of basting is to add flavour. But flavour is enough justification on its own.
Don’t use a spoon to baste — you’ll be all day at it. And many brushes are flops. Either the bristles melt if they come into contact with the hot sides of the roasting pans, or they shed out into the pan so that people are picking them out of their food. If you are going to baste, get yourself a turkey baster. As far as basting goes, it’s the only way to fly.
A baster has got a zillion other uses in the kitchen, in making gravy, in extracting a bit of liquid or water when you are cooking something and feel that you’ve overdone it on the liquid.
Basting is also done in barbequing. This is where basting brushes work great. But whatever you do, don’t use the same brush that you used to put a sauce on raw meat that you do to baste meat in the process of being cooked. That will cause contamination of the cooked meat. You need to either get two brushes, or just smear the sauce onto the raw meat with your hands, saving the clean brush for use during cooking.
Again, basting during barbequing or grilling doesn’t really make the meat any moister, but it does get flavour into it, especially because in barbequing you lose the drippings from the meat, and some barbeque sauces are almost works of art as complicated as French sauces. Some people claim that basting during barbequing prevents over-browning, but most people discredit that.
The absolute best way to baste a piece of meat is to put it in the oven right side up. When roasting a joint of meat, such as pork, beef or lamb, that has a layer of fat on one side, cook it fat side up so that as the fat renders the meat will baste itself.
Unfortunately, using bacon strips (streaky or American-style) doesn’t really work with fowl, because then the skin doesn’t brown under them. But you can put bacon or sausage as part of the stuffing inside.