Jeremy,
Thank you for posting, this is a good question. The RV120W requires an IP address change prior to setting up QuickVPN users. The reason is that the majority of Consumer and Small Business routers have a default LAN IP of 192.168.1.1. If a QuickVPN client is connecting from behind one of those routers they will not have access to resources behind the RV120W. The router that they are connecting from will try to keep all traffic destined for 192.168.1.x local instead of sending it through the tunnel.
You can change the LAN IP address to something like 192.168.2.1 or 192.168.10.1 if you wish. You do not need to use the 10.x.x.1 that the router suggests.
Let us know if you have any questions.