What are the items that an external GPS navigator (a la GTN650) can do that the AFS PFD cannot do?

My understanding is it has a WAAS GPS, has all the waypoints, ect.

Is it missing integrity monitoring and GPS approaches?

Could one (legally) fly an IFR flight plan using the internal flight plan in the EFIS and then switch to the external GPS navigator just to fly the approach?

-Craig