The purpose of drains is to flush wastewater out of the home, making the area a prime location for mold and bacteria buildup.
In the kitchen, food particles, food borne illness-causing germs, and other yucky stuff regularly get flushed down the pipes.
In bathrooms, you brush your teeth, wash your hands, and bathe your body, letting the residual water wash away.
Without regular cleaning, these areas can get pretty grimy. Buildup from oils, mold growth, and germs that love moist environments and can cause smells, look unsightly, and cause a real health threat over time.