It just means that you have 2 Gigabit ports on your motherboard... Useful if you are going to connect that computer to a router, and SHARE the internet connection through that computer to another computer... It would have no bearing on your ping times or whatnot, because logically, your internet is TYPICALLY less than 10Mb/sec, and Gigabit is obviously 1000Mb/sec... The bottleneck is off the wan port of the router, not in the LAN.
Useless? Do you know what latency is? It's the time it takes for data to get from your computer to the server and vise versa. It's hard to explain. Imagine this, your internet connection is a freeway with 10 lanes, each is one megabit per second. The latency is how fast the cars are driving.
OH, yeah dual ethernet is only good for when one goes out on you, or if one chipset isn't supported by Linux. But on my DFI LP SLI-DR, they both are. The Marvell one went out after I used Windows on it, go figure.