There is. Pouring hot milk or cream over chocolate pieces is a much gentler way of melting it. If you boil the chocolate, however, chances are good that you will overheat it.
Chocolate has a low melting temperature, and a low burning temperature. Boiling your chocolate for too long (more than a few seconds) will burn it, changing the taste and texture from sweet and smooth to bitter and grainy.
Simply pouring hot dairy over the chocolate allows it to melt much more slowly, preserving its taste and texture. It is the superior method, which is why most recipes advise doing it that way. If you're concerned about having more washing up to do (I am!) then you can simply add the chocolate to the hot milk off the heat and swirl briefly to disperse, instead of pouring the liquid over the chocolate into a second bowl; it gives the same effect.