Internally, G+ is marketed as a unified login/account system. Management's stated reasoning goes something like: * It's silly to force users to have separate accounts for Google services because most users would prefer to have Gmail, Docs/Drive, YouTube, Calendar, and so on under the same account/identity. * Users who want to have separate public identities will create pseudonymous "Pages" for each of their identitie…
Why doesn't Google provide a dashboard for all their services accessible from every Google service when you're signed in? The Dashboard presents all of Google's services with the ability to opt-in to additional services.
So, for example, if I sign up for a GMail account, my dashboard only shows I've activated the GMail service, but also lists all the other Google services such as Google+, Docs, Drive etc. You can use the dashboard to promote your services rather than cluttering the GMail interface with promos for Google+, Google Chat, Hangouts etc. It's a bit ironic that one of the stated goals of Gmail is to minimise distractions and yet the GMail interface is stuffed with promos for other Google services.