Hi Janneke
Check out the websites for Garmin Forerunner or Polar to get an idea of what other stuff they offer. It does go beyond just the gps, although that's the main part.
Things that I find useful / important are -
- You can program in workouts & courses that help you do, say, a number of fast reps with some recovery time
- On mine you can race against a "training buddy" - the watch display shows a little man either behind you or ahead depending on whether you are faster than last time (although apparently he gets annoying because he keeps beating you

)
- You can download your exact time & distance to your computer and see where you were fast or slow
- Stores months worth of data so you can track your progress
- You can see on a map or Google Earth exactly where you ran
- Long battery life & fast recharge time, ideally it can recharge from your computer
- Doesn't lose the signal when you're running under trees / bridges
- Finds the satellites quickly (mine can be annoying here)
Hope that helps!