[kennell]

The current real Mac Pro actually has two ethernet ports too. There are a number of ways this could be useful.

Number one would obviously be to connect to two different networks. It could also be useful in a scenario where the machine is used as a server and you want a semi-redundant backup network line. It could also be used to directly connect to a NAS box via Ethernet.

[skiril]

you can bridge network cards to double your connection speed. You can connect to 2 different networks (load balancing or 2 different Internet Server Providers). You can run multiple virtual machines on your mac and separate traffic instead of putting all into one adapter. Multiple LAN is a norm now, especially if you in IT.

[corn13read]

Bridged gigabit for twice the bandwidth