Variable apertures are helpful to enable us to control exposure but they're not absolutely necessary. We could make perfectly good exposures with just a single aperture and variable shutter speeds. In fact, that's exactly how a pin hole camera works. It has just a single aperture - the pin hole - and variable shutter, ie the length of time you uncover the pin hole. So variable apertures are not essential for getting good exposure, just helpful. No, what apertures are really essential for is varying and controlling depth of field - that zone of sharpness we get when we focus a lens on a subject. That's the creative power of apertures and it's using that creative power to enhance our photographs that assignment 2 is all about.
The idea is to create two photographs that really use the creative possibilities of depth of field in a meaningful, thoughtful and eye-catching way. It's a chance to show off your technical ability (look what I can do with depth of field!) and expand your creative horizons (what happens if I use a really short depth of field with this subject?)