Camshaft position senors and crankshaft position sensors basically do similar jobs, synchronize the engine motion for the ECM. However, crankshaft position sensors cannot determine TDC on their own. The crankshaft makes two rotations per cycle, so each vain in the crankshaft sensor wheel will pass twice. The camshaft(s) turns exactly half a fast, so they rotate once per cycle, making it possible to detect TDC.
I’ve seen engine management systems use either one as a primary. But the systems that rely on the crankshaft sensor tend to use a ‘wasted spark’ ignition system, where the coil fires two cylinders at a time, one on the power stroke, and the other on the exhaust stroke of the opposite cylinder. The EFI is non-sequential in this case.
Those that use the camshaft sensor can provide true sequential fuel injection, since they can detect the intake stroke for each individual cylinder, and can use an ignition system that fires one coil per cylinder, since it can detect the power stroke for each individual cylinder.
Many car systems use both, but typically only rely on one as a primary sensor, and the other as a back-up or error checking sensor.