Just like you don't want water from a clogged drain sitting on the floor, leaking into walls, or going other places - you don't want rainwater to do the same thing to the exterior of your home - which it will if the gutters are clogged. Gutters also need to be cleaned to prevent pests like mosquitos and rodents from making a home in your gutters and to help prevent black streaks from forming on the outer surface of the gutters.