http://www.ti.com/lit/an/snva425/snva425.pdf

2) What does Rramp do? The resistor increase the slope of the ramp that is added to the emulated current ramp.  If your system is already stable then the additional ramp has the effect of moving the part closer to voltage mode.  Therefore if your design does not need the ramp for stability it doesnot need to be added.