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The real function of a cam pickup is to provide the ECU with engine phase information. Since the complete engine cycle of a 4 stroke is 720 crank degrees any pattern found on the crank will be repeated twice. Without this synchronization it wouldn't know where to fire your fuel injectors - is it the intake stroke or the exhaust stroke? It wouldn't know without the cam sync.
Cam signals are not the best for accurate engine timing so carmakers tend to rely more on the crank. It is definitely necessary for more advanced functions like misfire detection as the slop in a timing belt or timing chain would soak up the variations in crank speed that the ECU needs to detect.
Hope that helps.
-Michael
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