Gear/Belt reduction is not always necessary. It usually is done to obtain reasonable rapid traverse speed. For example, let's say I happen to have motors that can spin at 5000 RPM. Running these direct drive on a 10 TPI leadscrew would give me 500 IPM as my maximum speed. For most of us, that would be nice, but is a bit excessive. Something like 100 IPM would be nice, because it would allow greater torque on the drive screw, thus greater cutting forces. In this case, going with a 5:1 reduction would get you to the 100 IPM rapid speed.
However, had you thought about this in the beginning, and bought 1000 RPM motors, they could be direct drive and achieve the same thing.
Another thing to think about when choosing drive systems is whether or not this machine will be used in manual mode. I chose the belt drive option so that I could still attach handles and use the machine in manual mode quickly and easily. Since you're designing it, it's all up to you. Good luck!
