Answer:
Aperture (the physical size of the lens opening) and shutter speed together control the amount of exposure, the total light that is allowed to strike the film or sensor. You'd want the aperture and shutter speed interconnected to control either motion stopping power or depth of field (you have to choose one over the other).
For example: given a certain ISO and a fixed light level, assume that you or the camera have metered the scene and the amount of exposure is correct at, let's say, 1/125 second (shutter) at f/8.0 (aperture). Your shutter speed and aperture can be interconnected to get equivalent exposures at 1/250 @ f/5.6; or 1/500 @ f/4.0; or going the other way you'd get an equivalent exposure at 1/60 @ f/11 or 1/30 @ f/16.
All the exposures listed are equal, even though they all sound different. The aperture and shutter are interconnected in that, as the aperture gets larger to admit more light, the shutter speed gets faster to limit the amount of time the light is admitted.
So why bother? Because there are two other factors involved. One is what we call depth of field, which is defined as the area in front of and behind a subject focused upon that appears also to be sharp. Depth of field increases with smaller apertures (the f/8.0, f/11 and f/16 of the example). So if you're shooting a very tight close-up of a flower, where depth of field is very limited due to close focus, you might choose the 1/30 @ f/16 option, but at that slow shutter with a close up subject you might also want to mount the camera on a tripod.
But let's say you're shooting skateboarders at the park. You're focused fairly far away so depth of field isn't terribly important, and in fact you'd want the depth of field relatively shallow to emphasize the skateboarder in the air, where you'd be much more likely to freeze him at 1/500 @ f/4.0. (Remember that these are just arbitrary examples.)
On the other side of the coin, you don't want the shutter and aperture to be interconnected when the light level is changing, or the ISO, or both. Your hand held or in camera meter will select a different combination of shutter and aperture for a correct exposure, and if you have the option of controlling both, you can still select for greater depth of field or motion stopping.