Because sometimes the crank trigger on the back of the crank isn't indexed properly when it's pressed on. If you're using an aftermarket ECU you can change the crank offset so you're not advanced/retarded from what the ECU is commanding. Factory GM ecu's have a procedure called the CASE relearn which helps the ECU determine the offset when the crankshaft is changed.
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'71 Holden HQ Monaro - 3850lbs race weight, 400c/i - 11.4 @ 120
'66 Pontiac GTO - 389, 4 speed street cruiser
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