These two sensors are not always used for the same purpose. Obviously, reading each cam and crank shaft position gives more granular information to the computer for tuning of the engine.
According to Standard (manufacture of automotive engine management components);
The Camshaft sensor determines which cylinder is firing to establish injector synchronization and coil firing sequence in DIS systems. Crankshaft sensors set ignition timing, supply the RPM signal, and determine engine speed.