People who live near dangerous animals, for example, need to be able to shoot to defend themselves. When you are terrified (for example because that Grizzly is running towards you), you lose fine motor skills. You may not be able to hit with the first shot, which is all you get with a single-shot hunting rifle.
Of course, if you live in a safe urban/suburban environment where you can rely on animal control to take care of dangerous animals and the police to take care of dangerous criminals, you don’t need such a rifle. But not everybody has “safe environment with professionals working hard to keep it that way” privileges.