External antennas have been available for GPS units for a long time. Ten years ago, it was a good addition to your GPS because the external antenna had a higher gain than the one internal to your GPS. Over the years that has changed and people now use external antennas because they would like to keep the GPS where it cannot see the sky. For instance, there are people who would like to mount their GPS using a seat bolt mount or a cup holder mount. While we find sunroofs help immensely with these out of the way locations, an external antenna is sometimes deployed to help the GPS reception.
There are different types of antenna connections. Most current car GPS antenna connections use a connector called MCX. You will normally find the port on the back or bottom of the GPS. While most car GPS units use MCX, many marine GPS units use a connector called BNC. Larger than the MCX, the BNC antenna connections are also typically found on the back of the unit.
Once you decide an antenna is needed, the next question might be where to put it. Some like to run the wire through a window and place it on the roof of the car. Since most antennas have a magnetic bottom, it will stay where you place it. However from a bad guy perspective, an antenna sitting on your roof is a good way to alert them if a potential target. Consider an antenna mount inside your vehicle.
Most antennas have a gain of roughly 30dB and can be found for less than $20 and are sufficient for everyday driving. However there are more powerful antennas that can double the gain, although at a much higher price.
So if you want to stash your GPS in a location different from the tranditional windshield but have some reception issues, the addition of an antenna might be what you need.