After cleaning your block paved driveway or patio yes you need to resand it. Using kiln dried sand on a dry day and the surface must be bone dry you can simply sweep the sand into the joints. The reason for this is a portion of the existing sand will be washed out during the clean. The purpose for this sand it to fill any gaps and voids between the blocks which in turn stabilizes the block and prevents them from moving in place which in long term would start causing an uneven surface in the blocks.