Why a resitor?

I am not sure i fully understand why do I need a resistor for my ramps if I am using only the cnc and not the 3d printing ext ruder??
what can i expect if i leave this out:

I have left the code in for 3D printer use. The firmware looks for a minimum temp as a safety check. You can edit the firmware to get rid of it, but really a resistor is really easy and you can throw a resistor on the machine somewhere to monitor temps.

If you leave it off without editing the firmware, your machine will not move.