Testing React Components renderedComponenent.length.toEqual(1)

I was wondering why the following code is considered as a valid test on Reactjs:

const children = (<h1>Test</h1>);
const renderComponent = (props = {}) => shallow(
  <Button {...props}>
    {children}
  </Button>
);

describe("<Button>", () => {
   it("should render a <Button> without a theme", () => {
      const renderedComponent = renderComponent({
        nature: "primary"
      });
      expect(renderedComponent.length).toEqual(1);
    });
 });

Why do we expect the rendered Component.length to equal 1?