If you look into the mechanics of the rear brake you will see that as brake pressure is applied, the brake plate tends to rotate downward. This effectively applies more brake pressure inasmuch as the brake rod stays in whatever position it was when braking. Te brake grabs and a shuttering takes place as the brake vacillates as the pressure varies.
The rod you mentioned tends to stabilize the brake plate permitting smoother action. Far end of the rod is bolted to the engine boss that passes through the original crank hanger.
The reinforcement plate stops the frame tube from bending. Depending on the frame used, actual breakage can take place if constructed from moderate wall thickness tubes.
The rod is a must for safe stopping. The reinforcement plate is your option but highly recommended. Picture shown is a bit "overkill". Remember to cut the slot in the plate to allow for the rear wheel to move for-and-aft to compensate for chain stretching.....which will be significant as mileage piles up.
Plate shown is large as machine has "side-car" for hauling heavy merchandise. Bike is property of Carl M. St. Louis, MO. and displayed at the Classic Motorcycles Museum in the same city. Well worth a visit.