Google Admin is a tool geared specifically towards the administrators of Google business accounts. That is, it's an app that only the 'superusers' of Google can use, not users with normal Gmail accounts.
Users can manage almost all of the important aspects of their accounts from the application: adding users, managing groups, resetting passwords, contacting technical support or taking a look at the domain options.
Google Admin is a very interesting application for Google 'superusers,' thanks to which they can much more easily and comfortably manage their accounts.