One or more of the following reasons:1)To offset the propeller from the centerline of the crankshaft2)To reverse the direction of spin of the propeller3)To increase the propeller's RPMs or torque, one or the other4) As an exercise in engineering design
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It's usually to decrease the rpms and gain torque so you can put on a bigger prop that will pull more air and be more efficient. I've seen rc planes running very wide props that you could almost count the rpms.
+1Many modern motors rely on high rpm. In order to develop sufficient torque, one needs to reduce the RPM
perhaps it will increase the turn rpm of turbines or increase the torque lowering the rpm depending on your motor