You get VoLTE. In a nutshell, better audio, possible video calls, and simultaneous voice and data as long as you're in place with LTE service.
I can't post links as a new member, but if you google "advanced calling" and "Verizon" it's pretty much the first result.
Some people have reported more problems with dropped calls (there's no handoff to the old cell networks if you leave an area with LTE service while on a call, e.g., driving) but as someone who spends 95% of my time in city limits, I haven't had this problem.
You also need to enable it in your account on the Verizon account site, btw. Once you've been provisioned for it (using My Verizon), there's a toggle available right in the Settings app on your phone to enable/disable as you see fit.