It's primarily if you want to run a separate LAN network at home, like using NAS storage, but using the other port for internet connection only.
Windows itself does not lend itself easily for "aggregation", you cannot double any Internet speed, though with an elaborate home network, you could improve local LAN speeds if you have "elaborate" home routers.
Put it this way, in simple terms - 99% of the home users will never use that second port. One port running at 1Gigabit speeds, will give the most speed most people will use, as almost all home networking is just 100mbps or 1gbps. Very few have anything faster unless they are "network geeks"
Now, if you build a server out of that board, it makes more sense to have a dual NIC - one for the "outside" and one for the LAN, using the server as a router/bridge between the two networks. Or, you can build a Linux firewall with one of those boards, giving you a very powerful firewall...
Source(s):
I have a dual ethernet motherboard - I just use the one port, and I run like 6 routers, wireless bridges and different servers at home over 5 public IP addresses....
