There is a widespread belief that browning the surface of meats seals in moisture and allows the final creation to be juicier than if it were not immediately browned. The idea is that the caramelization of sugars in the surface of meats creates a "crust" that keeps evaporating moisture from escaping.

From Le Cordon Bleu's Professional Cooking (a standard introductory textbook in many culinary schools):

"This does not actually happen. Meat does not have pores but is an open network of fibres. Think of the surface of a steak as resembling the cut end of a thick rope. It is true that heavy browning creates a kind of crust on the surface of the meat, but this crust is no more waterproof than an unbrowned surface."

It's really quite obvious why this must be true. Consider brown braising, which is the process of cooking an item partially submerged in a liquid after it is browned. The principle behind braising--tenderizing through the absorption of moisture into connective tissues--could not possibly work if browning were done first. The waterproof "seal" would logically allow no water to enter, and therefore the technique would not work.

On a more extreme note, it's plausible that if meats were sealable in this way, then we'd be in grave danger of meat exploding from water pressure, ala a baked potato.
