One good reason for fiddling with the combine is because the same factory machine is expected to harvest everything, corn, wheat, oats, soybeans, grass, canola ect. Kinda a "jack of all trades, master of none," and if your only doing one or two of these crops chances are you can tweak the machine to do a better job in your main crops. CIH has required different rotors to handle green stalk beans and other tough crops for years, then it needs tweaking to shell corn like the standard rotor. There is a bunch of aftermarket gadgets available for CIH machines. And I am sure the other rotaries need to be tweaked to better handle certain crops.