This keeps the satellite in it's position. A combination of forces trying to equal out to a stable area. 22,300 miles over the equator there is a whole belt of satellites zooming along. This area is named after Sir Dr. Arthur C. Clarke the science-fiction novelist. Science Fiction indeed.
Satellites are expensive.
In addition, we don't want to keep trying to find satellites every day. So we keep them at fixed positions so once we're locked in and pointed at the satellite we don't have to worry about tracking it across the sky. It's that simple. There are other reasons as well, but those are the most important ones. If you think about it, you'll realize that from where you are standing right now, you can see some things that are farther away from you than other things.