Sometimes sewer & water lines to & from the house get clogged. Roots may grow through the lines & using an auger or water jetter machine may not be able to unclog the drain. In cases of older houses in which they have copper or galvanized steel pipes may eventually, "rust out", causing water lines to flow slower & drains as well. So therefore the pipes must be replaced. This is why it is good to have a maintenace plan to clean drains at least annually with a Water Jetting Machine, not toxic cleaning products that can cause pipes to, "rust out" prematurely.