Why is it necessary when we already have an appliance label and a home Energy Performance Certificate both of which are based on the well understood A-G scale?

You are correct to state that in the UK we have had an energy label for some years, SEDBUK or SAP 2009. This brings the rest of Europe up to the same level as the UK and as members of the EC we have no say in whether we take it or not. The additional benefit of ErP ratings is for customers to be able to compare across different technologies rather than just boilers.