im not sure what the "offical" name of the part is, but use the wheel mounting stud supplied in the tire kit to make changing easier. its a 3 or 4 inch long stud that you screw into the top hole to "hold" the load while you apply the other bolts. then replace the stud with the last bolt. it really helps!

as to why: it may relate to MB wheels being "hubcentric". basically the wheel loading is held by the center bore, and not just by the bolts (or lugs)