Your Surfprep sander has either 3 or 4 speeds depending on the model. The sander is generally used on the higher speed but, the lower speeds can be used while performing various other sanding tasks. The primary reason you might want to use a slower speed is to better control your rate of cut, or finish, removal while sanding. Other uses for a lower speed would be for better control when working on delicate trim pieces on furniture or when working in a confined area with space and movement limitations. For those that use their tool for distressing painted furniture, the lower speeds are great for better control of your sander while doing so. I often will use the middle speed when sanding filler so as not to remove too much too fast. In summary, the lower speeds are used for better control and slower removal of material when using your sander.