Personally, I use mail from Google directly in the browser. There you can arrange a convenient aggregation.
Certainly such an app would be useful for those users who use third-party services without forwarding, or if the person is problematic to configure them (again, if the app is quite simple and intuitive).
Among the functions, vskidku, POPs up to automatically determine who came up with the account and to send different signatures, forwarding emails from one account to another (if you walked away from the computer so that you all the letters to read from the phone with a separate box). Well, everything else that he can a good email, only by under accounts.