Many GPS devices come with a heart rate monitor, and this allows you to check progress over time. For example as you become fitter your heart rate should be lower for a given speed, or you should be able to run faster for the same heart rate.
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. On a hilly run you will run faster going down hill but slower going up hill. Just having heart rate data won’t tell you as much as seeing this in combination with the profile of your run.
Most GPS watches will allow you to map your route on the computer after your session.
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.
You don’t need a separate bike computer if you have a GPS running watch – it measures everything you need on the bike. You can usually buy an attachment for your bike so you can put your GPS watch on the handlebars for easy viewing.
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.