By having two ports you can use your computer as a server... for example.. plug the internet into one and plug a switch/hub into the other, and you can set up your computer as a DHCP, File Sharing, and Internet Connection server/router all at the same time.
To be honest, it's 99.9% useless to the average computer user... and more of a 'filler' to justify the $300+ price tag than anything...