why is two better than one?

Why do motherboards have two ethernet jacks?
I have one hooked up to my router which connects me broadband to the internet. What would/could the second one be used for?

The reason I ask is because reviewers of the new i7 intel motherboard complain about there only being one port. So I thought, what's the big deal?

From the sticky ... second to last item.
How can I use two network adapters on one machine?
It is possible to use multiple network connections on the same computer. This is commonly done in the server environment, where computers will have internal and external connections (such as a mail server), however it is less common in the home environment. What you are doing is turning your computer into a router, and in many cases, it is easier and cheaper to buy a router/switch.
Why would someone use two network connections on a single machine? LAN Party goers may want to play on the internal network, while downloading a file from the Internet. You can also use a two connection PC as a quick and dirty server or firewall http://www.tomsnetworking.com/Revi [...] NOWALL.php see this article]. With one wireless connection and wired connection, your PC can operate as a wireless hotspot.
If you do decide to use two network connections on the same PC, then you will need to configure the IP Addresses of both connections. Each connection must be on a different subnet. Example would be one card at 192.168.1.1/24 and the other at 192.168.2.1/24. Then you have to assign preferred routes in the Operating System. In Windows this is done with either the command line ROUTE ADD command or by going to the advanced TCP/IP Network Properties in your network connection.