It seems to really be the strategy that was discussed early on for Google+ which was hampered by the existence of a special G+ app and, moreso, the need for an "enhanced" G+ account: the social layer is tying together existing Google accounts and products, with added social functionality both in the individual products and in new products which tie them together in new ways.

Even FB has given up the "one app to rule them all" approach to social, and Google was never very good at that.