I've occasionally read, including in Ansel Adams' writings, that viewing a print from the same distance as the lens' focal length (multiplied by enlargement factor if applicable) provides the most appropriate perspective.

What is the basis for this idea? I'm just asking out of curiosity. Why does the focal length of a lens determine how far away we should stand from a print?

And I also don't understand why this rationale is used in support of LF and ULF contact prints, whereas the same image / final print size should demand the same viewing distance irrespective of format (i.e. a format that requires 2x as much enlargement will require 1/2 the focal length -- so the viewing distance will be identical in the end).