Because the sewer lines are underground, we often don't consider them when walking through the home. Especially in an older home, a sewer repair can be a large and unexpected expense and a surprise that nobody enjoys. Older cast iron and clay lines are past their lifespans and susceptible to cracking and corrosion that make them great places for tree roots to look for water.
Even newer homes can be at risk. Although building materials have improved, plastic plumbing waste lines are not immune to hidden damage, installation error, or everyday clogs. These conditions can often leave the line appearing functional at the time of purchase, but will deteriorate over time.