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They just protect the shock absorber, particularly the shaft and the seal where it moves in and out of the shock body, from dirt. Too much dirt and grime will compromise the seal over time. Although, they do not fit snugly over the shock and on mine just kinda sit there as the shock boot is way larger than the shock body. But better than nothing I guess. They are about $15-$20 from what I remember. One of my rear ones were torn.