What's the Point of Having a (Slippery) Glass Back?

You are not the only person who prefers the Note 4 backing. I'm running the Note 5 but periodically go back to the Note 4 because of the removable battery. I use a case on all my phones so the glass back has no utility for me.
 
As far as the styling and looks - if it wasn't also for the wireless charging - Samsung was basically forced by the media and masses to go this route to make the phone feel more "premium" because so many people commented about how non-premium the plastic backs felt. I say "forced" tongue in cheek, but one of the top criticisms of Samsung Galaxy phones during the earlier iterations was that it did not feel premium in the hand.