Compass in mobile phone is in parallel plane to mobile (In the diagram X-Y plane)
Compass in all 3 planes
we need in other planes too (Y-Z, X-Z planes). (Alternatively, have a 3D compass which operates in all 3 directions/planes)
While in Navigation, mobile won't be able to use it's compass in X-Y plane to identify if the car took turn or not. current mechanism is only by joining multiple position dots. but not really a turn recognition (turn @ single position can't be detected).
for a vertical mount of mobile (as shown in photo) in car, compass in X-Z plane helps (for horizonatally mounted mobile, Y-Z compass helps) identify if the car made a turn/not. This helps in quickly re-routing.