Most companies who sell Fully-Qualified Domain Names will require the subscriber to have at least two public IP addresses, which are assigned by an ISP, for a Primary and Secondary Domain Name System. Ideally, these would be on (at least) two seperate computers with two seperate accounts with seperate power supplies to decrease the chance of the domain name being down entirely.

Most DIY'ers don't have two computers to do this with, or two different ISP accounts. Since both IP addresses (primary and secondary) will be coming into the same cable into the same NIC, then the computer has to be told to listen for both IP addresses on the same NIC (ie. MAC address), thus a virtual NIC.

If you have two different ISP accounts coming into two different NIC's, then this is not needed, you would assign the NICs as normal (eth0 and eth1, for example).