If the ECU knows Crank position, it knows where are the pistons. Without Cam position, the ECU does not know if the piston is on intake, for example, or power stroke -- it only knows the piston is on the way down (from the Crank signal). So the Cam position signal lets the ECU know what stroke the pistons are on.
I don't know details of this engine, but having Cam position would be useful for misfire detection, possibly for synchronizing fuel injection timing with the intake stroke on each cylinder, firing the right coil-on-plug ignition, etc.
Not surprising it can 'limp mode' just fine without the cam sensor, though, because the ECU can just guess one of 2 possibilities and if wrong guess the second.