Good question! I'm not sure the correct terminology what you describe is "focus" stacking, but it is stacking definitely (I think Exposure stacking might be more appropriate). Landscape, as you probably know, you tend to stack to increase dynamic range, so merge images at different exposures to retain information in highlights and shadowed areas, but you don't move the focus point. Astrophotography, you stack multiple images to reduce noise and increase the resolution/detail than you could ordinarily capture with a single image on your sensor. Again, you don't move focus, which for astro will be somewhere around infinity in every frame.
With macro, or indeed closeup photography (which this is an example of) you tend to stack images to increase the depth of field you would otherwise achieve in a single image, especially at something like F2.8. This is true "focus stacking" in the sense that you move your focus point between frames, to increase the overall area that is in focus. In the image above, have a look at the butterfly. It's all in focus, but with a single image I would expect to usually see part of the wing that was slightly blurred (even though it is facing us side on). Certainly I would expect one of the antenna to be outside the field of focus. It's more obvious when you look at those seed heads, even when stopping down the aperture a fair bit, it would be difficult to bring it all in focus in that way. Unfortunately stopping down the aperture further to increase your depth of field isn't always the right solution. At some point you will begin to see a drop in detail as the light entering the lens is forced to bend more through a smaller aperture. This is a phenomenon known as diffraction and is another reason why macro photographers choose to stack images. It get's more complicated as the amount of depth of field and the level of diffraction are affected by the level of magnification (it only gets worse as you get closer).
Hope that makes a bit of sense, or at least gives your some ideas for further reading. As you can imagine, it quite a large part of macro photography and there are a few details to understand around it , but it mainly comes down to increasing DoF and retaining detail.
@eldruida great image and a very fine stack
Edit: I should point out that often focus stacking is used in landscape to achieve great DoF across huge distances, but it is rarely a requirement with wide angle lenses that can achieve great depth of field at quite low apertures (by macro standards). I have a wide angle where I can get from about 2ft to infinity in focus at F8. If I'm shooting a subject at 1-to-1 magnification I'm lucky to get more than a centimetre in focus at that aperture.
