This board provides an additional 2 outputs for fans, LED lighting etc. on a RAMPS controller
If you are using RAMPS for a dual extruder setup then you will soon discover you need all 3 of the hi current outputs to run the heated bed and two extruders. So where to connect the fan?
Or perhaps you want to add some LED illumination to the print head.
Here's the solution. This add-on plugs into unused outputs on the RAMPS board and provides 2 additional high current PWM outputs.