It's mostly to do with noise and strength.
The "standard" and "reverse cut" gears are essentially the same thing which is a type of helical cut gear. Helical cut (stock E50 style) gears are generally stronger and quieter operating with less gear whine, however there's axial loading which robs an almost negligible amount of power while putting side pressure on the bearings and cases.
Straight cut gears are generally lighter and directly transfer more of the power than helical cuts but tend to be noisier and not as strong. (edited)