I just watched a Park tool video on cantilever brake adjustment. The mechanic pointed to a small screw and said it was to center the brake pads but did not say why. Obviously two forces that oppose each other should be applied evenly. My question is,can uneven application of the brake pads lead to a problem other than inefficient braking.
On rim brake calipers the cable pulls the two sides towards each other to 'pinch' the rim. If one pad contacts the rim first it will not exert any significant force until the opposite pad contacts the rim.
Centering the calipers allows the distance between the pads and rim to be minimized, thereby reducing the distance the lever has to be moved before braking force starts to be exerted.
Generally when brake pads are set up, there should be very little clearance between the brake pad and the rim. If the brakes aren't centered properly, there will usually be one pad dragging against the rim causing unnecessary wear on the pads and the bike to be inefficient to ride. They don't have to be perfectly centered, but it's usually the case that if they aren't centered well, then they will end up dragging after a short period of time.