I won't argue the case of it keeping the meat moist (though I don't completely agree) I will say that it's a primary factor in the forming of a good bark; and the smoke ring, if you are in to that sort of thing. A mop, different from a spray, a spritz, or a glaze, allows you to get large quantities of liquid and spice onto your cooking medium without disturbing your rub. A spray/spritz usually doesn't have spice (because it would clog the nozzle) and a glaze is thicker and requires a brush, which will disturb the rub if applied to early. Not to mention burn as the a "glaze" implies a sweetener of some sort which has a low burn temp.
As to what is in the mop, well that depends on what you are cooking. For pork, I use a combo of some sort of juice (usually apple) and hard alcohol (usually rum) and it is usually in the form of a spritz, not a mop. For beef I use mostly beer, salt, and pepper.
One last thing, if you are collecting and planning to reuse the drippings (and you should) make sure you don't over do the mop. If too much mop sauce lands in the drippings, it can change the end flavor of the drippings; usually for the bad.