If you have an older home you may have a combination of drain pipe materials in the home. Often times these are joined together with rubber couplers. When these are used they are required to also have a stainless steel band present around the rubber portion which is held on by the straps. This reinforces the rubber material and lessens the chance of deformation of the coupler. For vent lines, the rubber ones alone are acceptable as the vent lines do not have water in them. This is a very common defect I find in plumbing work, especially in older homes.