I just saw a claim to this end in another thread, and it is perpetuated about the internet and in many cookbooks.
So: Is it true? Does making a nice crust (maillard reaction) on a roast seal in juice and yield a juicer end product?
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No. As you noted, searing beef performs what's called the Maillard Process (or Reaction) which is a specific form of caramelization. Nothing is "sealed" into the meat because the meat isn't sealed by the process. It's still porous and will therefore leech moisture during cooking. You can retain moisture in cooked beef by buying quality beef and not cooking it beyond medium-rare to medium.
Alton Brown did an experiment in an episode of Good Eats called "Myth Smashers". http://www.youtube.com/watch?v=AW9npAc2Sgw
If you are measuring the overall progress by internal temperature, then searing the outside will not result in juicier meat.
However, if you are new to cooking and trying to measure doneness by the outward appearance you see at a steakhouse, then by the time the steak looks "done enough", it will probably bone dry inside.
On the other hand, there is usually a reason behind most myths. It might be that the savory result of the crust combined the greater contrast between the texture of the crust and the center of the meat makes your brain interpret it as juicier.
What matters most is what you enjoy, so cook one steak with searing and another without. Do a blind taste test and see which one you actually like more.
Searing meat is beneficial for developing color (color = flavor in cooking) and for "jump-starting" the cooking process.
As noted in the previous answers the more browning and crusting (within reason) that you develop the more flavorful the meat will be. A good experiment to compare the difference that browning has on the flavor of food is to saute a piece of chicken breast and poach another chicken breast. The sauteed will have a richer "meatier" flavor than the poached one.
Searing also "jump-starts" the cooking process by quickly transferring the heat to the interior of the meat so that it will take less time when roasting in the oven. If a large roast is simply seasoned and placed in a preheated oven the exterior of the meat has to first heat up before the heat is conveyed to the interior. If another roast of the same size is first seared and then transferred to the oven at the same time the first one is put in, the second roast will reach its finished desired temperature quicker because the exterior has already been heated and the oven heat will continue to keep the exterior temp. elevated to quicker convey to the center and cook.
Bill Buford in [HEAT] discussed this, and came up with the conclusion that we brown meat simply because it tastes better.
There are many fallacies that have crept into our kitchen culture because of careless cookbook authors. For example, The Joy of Cooking is responsible for the American need to rinse off pasta before saucing. (Something you should never! do).
As well, it has been shown that there is no harm in washing mushrooms, other than the hot fat/water splatter if you don't pat them dry.