@phil.davis:
For site-to-site links connecting private subnets at multiple locations, and servers for road-warriors connecting in, then you don't need an interface assigned. You can do it all with ordinary OpenVPN config - putting private subnets in the appropriate "local network" and "remote network" fields of the GUI, adding client-specific overrides for site-to-site with multiple clients from remote sites connecting in to 1 server… The GUI fields result in the necessary routes being created, then you use the general OpenVPN tab to allow traffic - often you only want/need to allow traffic between your various private IP subnets.
As doktornoktor says, if you are OpenVPNing out to a server somewhere for general internet access, then you probably need to add a gateway on the link, and direct certain (or all) public internet traffic over the link... and that needs the interface assigned.
If you are providing roadwarrior access with openvpn, you could use squid and squidguard to speed up your connections, so in this case you need the interface also assigned.