I have a Samyang F/2.0 12 mm for the Sony E Mount, so the 35-mm equivalent focal length is 18 mm (crop sensor 1.5×). I use it mainly for landscapes and architecture in order to fit more in my frame than I can with my other lens, usually focussing close to infinity (I also use it for photography of starry nights such as the milky way or star trails, as it is also by far my fastest lens). I sometimes use it in small indoor spaces, with a focus perhaps down to 1–2 metre at the lowest. But I've never used the lower half of the focus ring, which includes focus down to 20 cm. When I want detail, I tend to use my zoomlens (18–200 mm). Under what circumstances would one use a wide angle lens with a focus less than (say) 50 cm?
In the example below I took different photos of the same object, first twice with my Samyang 12 mm prime lense, then three times with my Sony E 18-200mm zoom lens, at various object and focal distances. I downsampled the images from 4000 to 1200 pixels wide but otherwise did not do any other post-processing than in-camera processing. The candle holder is about 9 cm high and 12 cm wide.
To me, the photos with the zoom lens appear more interesting for this purpose. The bokeh is stronger (even though all photos were taken at F/8.0) and the object fills more of the frame. What possibilities does an ultra-wide lens offer when I focus on an object that is close by?