I would love it, though, if PDF included this as something that's always entirely optional.

Sometimes I want to read something just as formatted. If my display is big enough, and the formatting has any importance, I probably want that.

Other times, my screen is smaller (phone) or the wrong shape (small laptop), and I'd rather the text confirm to the device rather than vice versa.

Also, sometimes I use my arrow keys to scroll as I read to keep my place (like a line-oriented instead of page-oriented bookmark). So just because the device is capable of it, I don't necessarily always want page-oriented original formatting because it might have two-column text to deal with or top/bottom margins that serve no purpose for me.