It's probably one of those weird french rules, but the point is mostly to allow the meat to steam and not boil, leading to more breakdown of connective tissue. At least that's what they said at culinary school.

I'm sure you can find hundreds of different ways of doing it depending on the cuisine, but I find that to be a good rule of thumb, since the more you submerge the meat, the more browning from the initial searing is lost to the liquid. Better sauce but weaker meat.