UI specifications are inherently incomplete and often require feedback from stakeholders. You're developing software for consumption by regular humans with feelings and emotions instead of for another computer or developer. Then there's the "implied" specifications which could be anything from current design fads, to legacy support, accessibility, usability, and plain old opinion.

Maybe a bad example, but it's basically mandatory to include a manpage for a cli utility, but do you always include drag & drop for a file uploader?