There are billions of old and ill small animals and birds each year who would normally be taken care by various predators. Around humans pretty much only cats can do that important and necessary job. The popular Smithsonian study that people like to cite which states billions of killed by cats somehow fails to mention what share of killed are ill/old vs healthy. There is no surprise though in such a glaring deficiency of that study once one learns that that Smithsonian team has a researcher convicted for animal, specifically toward cats, torture.
Another aspect - rats, a human civilization companion, raid nests for eggs thus decimating birds population around humans. By controlling rats cats help to maintain birds population.
