While old refrigerators often lacked a water line, you may find that most new ones come with a connector and hose to run a water line, much like you would from your home's laundry hookups to a washing machine. A look at the varied functions of modern refrigerators explains the need for a water line.
Modern refrigerators do more than old models. They can often make ice on their own, and higher end models also dispense cold and hot water.
Refrigerators need water lines so that they can get the water needed to make ice and dispense water. That being said, if your refrigerator doesn't have an ice maker or water dispenser, or if it has one or both of these but you prefer not to use them, you don't need to connect the refrigerator to a water line.
If you have a refrigerator connected to a water line for water dispensing or ice making, you'll find that little is required of you to ensure the appliance and water hookup work properly. If you want the refrigerator to make ice, you typically turn a knob or push a lever on the ice maker in the freezer. The refrigerator automatically gets the right amount of water needed to make ice cubes and continues to make ice cubes in batches until you turn or shut the ice maker to the off position. For water dispensing, pressing in a lever results in water traveling through your water lines to your refrigerator and coming out in a controlled stream until the same lever is depressed.
Hooking your refrigerator to your water valve is a simple, do-it-yourself project requiring few materials and tools. You'll need quarter-inch copper tubing that measures the length between your wall hookup and the hookup location on the back of your refrigerator, plus about 7 feet (to allow for moving the fridge). Drill a quarter-inch hole into the water pipe and attach a shut-off valve to the pipe using a clamp and packing nut to create a watertight seal. Slip on the copper tubing and attach with a compression nut. Place the other end of the tubing into a sink and then turn on the water to rinse out the tubing. Shut off the water valve and attach the open end of tubing to the valve on your refrigerator.