Some network protocols send frequent packets for management purposes. These can be routing updates or keep-alive messages. In a WAN this can introduce significant overhead, due to the typically smaller bandwidth of WAN connections.
Spoofing reduces the required bandwidth by having devices, such as bridges or routers, answer for the remote devices. This fools (spoofs) the LAN device into thinking the remote LAN is still connected, even though it's not. The spoofing saves the WAN bandwidth, because no packet is ever sent out on the WAN.
LAN protocols today do not yet accommodate spoofing easily.