Client machines may partake in two different network segments, and server machines may generate separate network segments.
In a simple home environment, you might for example have your main machine act as the gateway, firewall and proxy for the others - and for that, you'll be using one LAN port for the local wiring, and the other for the WAN connection to the outside world (the DSL modem typically). Want to establish a WLAN segment with different rights and firewalling rules? Add a third and hook the AP device there.