what i know u can have 2 lan cards but u can't use 2 connections at the same time then what is use of 2 lan cards.
Oh Noes !!
On a more serious mood, its quite usefull when you want to offload traffic. Or when you have several levels of priority for data transfers. The process is really similar (in a very strange way) to assign processes to the varios cores (counting that you have more than one).
You can use two NICs at the same time. Typically, each one goes on a different network, essentially turning your PC into a router.
For example, a PC with two NICs...one NIC connected to your cable modem, the other to your router. This PC could be running firewall software, for example.
Thats one good example. Other for example, related to gaming. I have a nice Amd 1500+ doing server work in my house. When i wanna make a lan and play (COD, CS or other LAN games) i just pop a virtual machine and those virtual machines are already using my eth1 ( second NIC ). Like this, i can share everything ( net, printer, etc) and use the second NIC to just gaming.
My house is strange. Record so far 16 computers. We couldnt plug more with out the electricity shuts down. Ehehehe. Good times.