Pet waste contains bacteria and can contribute to excessive algae growth, E. coli, and other water quality issues that can be harmful to people, pets, fish, and other wildlife.
When it rains, some of the poop left on your lawn can be washed into the nearest storm drain and carried to the nearby streams, lakes or rivers. Picking up pet waste, even in your own yard, keeps our local waters safe and clean for recreation and enjoyment.
Most cities have an ordinance making it unlawful to leave your dog's feces on any public property including parks, streets and sidewalks, as well as private property that is not owned by the pet owner. Google your city's pet waste ordinance for details.