I think dual gigabit LAN are for workstations and servers. One of them could be for the newer Socket 940 and 939 Opterons...
by having 2 LAN u can split the LAN cable from ur modem to the mobo n u double the speed of ur broadband an example would be if u hav 1mb line..split into 2..plug into the two LAN port n u hav 2mb