If you don't plot it, how do you know which heading to turn to, to make your destination?
The skills test requires me to arrive at the diversion point within +/- 3 minutes of my ETA. I need to draw the line so I can measure it and estimate the ETA based on my speed.
The south of England is all flat and looks very much the same. Fields, towns, fields, towns, fields, towns. You need to be more precise to find some disused grass strip in the middle of nowhere that the examiner wants you to find.