This biggest thing that comes to my mind is if your wanting to setup two different networks with the computer on both. Granted this isn't something that you would do normally, but is something that you could use every now and again. For instance, I have my desktop and laptop on the same desk. My desktop is plugged directly into my cable modem as I don't use my laptop at home and therefor don't keep it on the internet. Occassionaly I want to move files from my laptop to my desktop, which means with my current setup I have to recable my cable modem from my desktop to my router and then my router to my desktop and laptop to have them both on the same network; then I have to recable everything back when I'm done. If you have two ethernet ports on your desktop however, you could just run a single port to your modem for internet and the other port to any other PC's you may have for file sharing. Kind of pointless in my eyes for the typical consumer to have two ports but they could come in handy. I'm sure there's other applications for having two but none that come to my mind at the time.