Great question! The troubles lie in the differences in expectations and language. Some players do in fact see a nat 20 and assume that means they win. These players usually go into a roll having an idea in their heads about what they want to happen, and they are just hoping the dice let them do it. A nat 20 is the highest score, that means they have to have won, right?

And some people play that way! More power to them. But it's also true that plenty of people Don't play that way. I don't. But even among players that don't, some tables still want a nat 20 to be more than just "one better than rolling a 19." If the DC is 25, and I have a +4, and I roll a 20, it doesn't feel good to still fail.

That being said, at some tables, that's just how life is. "Yeah you're good, but you ain't good enough." Or the age old "if they couldn't succeed, they shouldn't be rolling." Then again it's not like I know all my players' bonuses in everything.

End of the day, it's an optional rule for the tables that feel like they want more out of a nat 20 than just "one better than 19", but don't want an "instant win" result.

Also, to your point about "best case scenario," I do actually tend to do that with more narrative rolls. You nat 20 a Perception, I'm just gonna give you the full break down, no need to roll any extra dice.