AFAIK they tend to help pads wear more evenly by keeping dirt off the disk and then the pad but it's marginal.
Not sure about clearances and tolerances in the rear but in the front if you leave them out the hub seal will run approx 1.5mm further inboard on the hub, can be an advantage if the old seal has gone hard and worn a groove in the hub.
Not sure about clearances and tolerances in the rear but in the front if you leave them out the hub seal will run approx 1.5mm further inboard on the hub, can be an advantage if the old seal has gone hard and worn a groove in the hub.