A key defining principle of the Canon EOS DSLR camera system was to make it simple and instinctive. Full-time manual focus for lenses with USM focus motors fits that remit. It is useful for subjects that don’t move a great deal; the ability to simply turn the lens’ focus ring if shooting macro or landscapes was so instinctive that you might not even have realized that it was a designed feature. It also means that the lens can be manually pre-focused before you even start the AF.
When EF lenses for DSLRs were introduced with fly-by-wire focus rings, new custom settings were added to the cameras to still enable focus to be adjusted slightly once One-Shot AF was complete.
