GPS watches allow you to monitor and control individual training sessions, as well as track your progress.
Many GPS devices come with a heart rate monitor, and this allows you to check progress over time.
GPS running watches are also useful for ensuring that over the course of a run, your pacing is appropriate. People often run too hard/fast on easy runs.
If your watch measures altitude this allows you to assess how much vertical ascent or descent was included in your run. This means you can assess the impact of your run more effectively.
Some GPS running watches have bike modes, and some have ANT+ technology so you can connect them to power meters, with all the data being collected on the GPS device.
Don’t worry about a GPS running watch being complicated to use, most of them are very simple, and a lot of them don’t even need to be attached to a computer to download.