Why do they do that.  It's called "trussing" a chicken.  But why?  Are chefs scared the chickens escape?  I hate that string.  Getting that string off baked chickens is like solving a Rubik's Cube.

The reason for the string (trussing) is to keep the breast cavity closed.  If the breast cavity remains wide open, too much air circulates inside of the chicken drying out the breast part before the thighs and legs are cooked.  It makes sense, but it sure is a pain.

Here is the proper way to truss a chicken for even cooking.