Questions: Why do we need to configure LAN and WAN (Internet) IP settings on home router?

Normally you are required to configure LAN and WAN (Internet) IP settings on home router, but you might wonder why?

The WAN (Internet) IP is used by your home router to communicate to Internet servers via Cable/DSL modem, whereas the LAN IP is acted as gateway IP on your home network, so that other computers can set the router's LAN IP as gateway IP and access to Internet via router. Usually the LAN IP is private IP (cannot be used on Internet), such as 192.168.x.x and 10.x.x.x. Then how can the home computer with private LAN IP access to Internet? Ha.. Because there is built-in NAT (Network Address Translation) on router that will translate the computer’s LAN IP to WAN Internet IP when accessing to Internet.