There's a lot of possibilities, for example:
- Connect it to multiple networks/computers
- Connect another computer to it and have them share a connection
- Create a hardware firewall out of the PC by connecting the modem in one port and a router in another port
Connecting to two networks is the primary. For example almost everywhere I go, theirs at least two seperate networks. Also sometimes you may want to use multiple IP addresses on the same network, for example a recent fileserver joined the domain under the IP address 10.145.32.14. However it's replacing the fileserver 10.145.32.12, but must still use it's 10.145.32.14 address. Adding a second network card with 10.145.32.12 will redirect all traffic from the old server to the new server.