The abrasive washers reduce the amount of torque required to hold the door hinges in place and thus reduce any strain on the door's fibreglass. The abrasive washers also greatly decrease the likelihood of hinge bodies slipping. You don't have to fit abrasive washers to the standard set-up either. But that does mean they may slip and the hinge mounts nuts must be much tighter, potentially adding strain to the the fibreglass.
The Banks hinges don't come with abrasive washers but they were work better with them. I made them up (easy to do) and used them with mine. The main body nut just needs to be nicely snugged and the door is rock solid.
