Camshaft and crankshaft position sensors

Hello.

I am confused over a question. Fuel injection timing, spark timing, and engine speed, all can be calculated using camshaft position signal only. Then why is a crankshaft sensor also needed?
Can anybody help me understand the need of two separate sensors when we already know the camshaft is driven by crankshaft by mechanical timing belt and both are inter-related?

Regards

Helps to determine mis-fire data