Few things are scarier than driving without being able to see. We have all been caught off guard by a sudden downpour and the windows suddenly fogging up reducing the visibility to near zero. They don't simply clear the glass for better vision in the rain and snow but also clear off dirt and debris that gather on the windshield. The windshield wiper arm is the actual metal part that attaches to the car compared the rubber part which is known as the windshield wiper blade. The arms do not need to be replaced as often as the blades but the windshield wiper arms can become rusty or break off in accidents like at an automatic car wash or moving them when they are covered with snow. Old wipers may leave streaks or missed areas all of the windshield or worse may not clean at all making it virtually impossible to see in heavy rain or sleet. This not only means you will have trouble driving in inclement weather but you will not be able to pass inspection as states require that cars have properly working windshield wipers in order to be road legal.
Early cars did not have windshield wiper arms at all but often had a vertical windscreen with an overhanging canopy to offer a little bit of shield from the weather. The first ones were manually operated. They are nearly always made from metal. Later more innovations came to their design with multiple speed wipers, automatic wipers and intermittent wipers. Many different types have been developed for hazardous conditions like extreme snow and mud for off-roading. Most cars have two wipers but for many years Mercedes-Benz had only one wiper on some of their models. Other cars like the Jaguar XKE had three windshield wiper arms. The arms are usually unequal in length. Today's car nearly always have the arms going from the bottom near the hood but some old cars had them suspended from the top.
Make sure you are in a clean well lit place. Some trucks and cars may need to have the hood opened in order to complete the job. There are several different styles of windshield wiper arms so it the instructions may vary. As always, seek professional help if you feel that you will not be able to install them on your own.