...Because you are thinking linearly. What if you wanted to trace the edge of a track to see how long it was? If the track wasn't just made of strait lines, then you would have a very tough time doing it.
Also, consider the implications of being able to see how far you went if you started and stopped in the same place. In that case, a pedometer would really come in handy to track the length of your path.
Be creative. If you HAVE to try and find a reason to shoot this down, you may be able to. I find that it is much more fun to imagine the possibilities.