It's useful when you don't want to have to wire a long cable again just to reach the other side where your router might be. Instead you can use the existing computer already wired as your gateway into the Internet. It saves from buying a long cable or when you don't want to buy a switch just to add one more computer to your network, also ICS with the wireless ad hoc is really useful just for this but it's unreliable and confusing for a beginner to setup.