I've just bought the Nikon 20mm f1.8G lens, having had a Voigtländer 20mm f/3.5 SL-II Aspherical (Nikon Fit) for years. I've also just written a post on how much I like the Fuji 14mm f/2.8 (35mm 'equivalent' of 21mm) So what is it about this focal length i like?
It is I guess, wide but just before things start to get very distorted. I do like ultra wide-angles, but shooting what I do, it's often difficult to get close enough to what I'm photographing to really make them work. A 20mm (or similar) focal length seems to suit what I shoot more than 14mm, 16mm or 18mm (35mm 'equivalent'). The 20mm Nikon lens also has it's attractions in that it's a fast lens and I do like the look of images shot with something like that. On my 'full-frame' Df it's also an advantage to get the most DOF I can and a 20mm helps with that while still retaining some semblance of recognisable perspective.
It's interesting that one of the lenses I used the most on my Pentax 645 film camera was a 35mm lens, which equates approximately once more to 20mm in 35mm terms. I shot 1000's of images with that lens. Medium-Format puts a premium on depth-of-field and if you struggle with 'full-frame' then just imagine the difficulties of getting enough DOF from something significantly bigger. Particularly since most of my travel pictures were shot hand-held using the 645 loaded with Fuji Velvia 50. You'll not be surprised to learn that I tended to work in very good light only, and I lost count of how many pictures I took with that 35mm lens at 1/60th. f/11. Even then I could still be struggling to get enough of the image in sharp focus.
So, there are historical reasons why I feel 'comfortable' with lenses like the 20mm. I'm used to composing with it and I have a good idea of what and how to frame with it. Anything wider and I'm entering unfamiliar territory and certainly when I fit my APS-C 10-18mm zoom onto my Sony FE cameras, it can be an interesting experience!!