1) It Works Like Swift When Mailing/Hangout Comes Along With It
2) Easy Video Calls No Registration Required
3) Google Chrome Profile(All History and Bookmarks Access Anywhere)
4) Google Drive Access(Access Your Cloud FIles From Anywhere With Gmail)
5) Lastly Google+ Profile(No Registration Required)
All 5 benefits are there for you if you choose to have google mail there are more like google photos and other applications that you can access directly with Gmail or Google plus respectively.