Put a shape behind a container that would otherwise be invisible against the page background.
This is mostly for visual interest. The fact that the container has the same background as the page means that it blends in better with the page as a whole. This means this technique is useful if you want to highlight a container but not so much that you’d use a high-contrast background colour for it.
I’ve used a heavily blurred shape in the example below, but there’s no reason you can’t use a solid shape.
There’s a nice background-merging effect to the sides of the container if you make sure the background shape doesn’t take up the full width of the screen.