So let's make sure we get some terms straight.
"Fencing" is when you have a Direct External Network assigned to the vApp, and you want to isolate it. A vApp network is similar, but the internal addresses differ from the external addresses.
In both cases and Edge device is deployed for isolation purposes.
If you disable FW/NAT, then the edge functions as a pure gateway. This means you could setup some static route to say that a specific network or IP range exists behind the external IP of the Edge device. This is more useful in the case of a vApp Network or Routed Organization Network.