They are the easiest and fastest way of connecting 2 or more comuters together. If you ever want to network your pc to play games (lan party!) or share files, you'll need one. Most companies use NICs for their networks.
NIC Cards are basically a modem to connect to other computers. They use similar technology to phone lines but the cable used to connect them can handle a much faster data rate than a regular phone line or 56k modem. Like the reply before mine. They are mostly used in LAN's (Local Area Network.) To connect multiple computers to the same internet connection or just to transfer data and share resources in one building.