why GPS?

for a few reasons:

First reason- in some positions of the Roadbooks you will have azimuths. That means that we give you an angle relative to the north (azimuth) and a distance on that direction so you can find the next position.
The distance is aerial (line of sight) calculated between the start point (the place you are) and the end point (the place you're looking for). This was done with the GPS and is very precise.
You will have to know how to operate your GPS in order to create a new waypoint based on the distance + angle we give you and then to go to that point. The route between the 2 points is not always direct so most of the time the distance on the ground (the one from the Trip meter) is longer.
If you use a compass you don't have enough precision on the angle reading and you cannot estimate the correct distance (line of sight).

Second reason- it's for your safety. In case you are lost or the car is broken, the only way to describe your position is by the GPS position.