What is the difference between restoring and installing an app on an Android device?
Well, one that comes to mind - if application is removed from market or is not listed as compatible for your device, restoring from Google account is one way of getting those applications. I did it with "FML official" and "Amazon" applications when they were not available on Nexus 7 2013 - "backed" them up on my Samsung S2 and "restored" on Nexus 7.

And, of course, for some applications settings are restored too.