[TrainedMonkey]

I think while that is one benefit of their move, it was not primary purpose. Google is data. They had different users on different services, now they are trying for unified account (Which is probably going to be G+ with everything else attached to it). In essence that is to allow people "convenient" access to all Google services with a single login. However, really it is to tie real people to their services (Easier to sell shit to real people) and to chain them to using Google.

I know there are lots of other reasons, but I believe what I stated above to be main reasons that outline Google's long term strategic plan. Keep driving ad click through rates up by showing more and more custom ads to everyone.