Great reply, educational . . . thank you.
-- it could be that over years of using this technology folks(ie ME) had gotten used to a certain understanding but while the technology morphed from manual "syncing" to automagic iCloud synching, along with making ALL devices controllers of the data, many occasional and new users lost track of the program. It is not a hard concept to understand, like you say, but preconceived notions affect one's ability to assimilate the new stuff.
-- a problem in the concept, again for me, is that I have zero interest in having multi-device access to my ENTIRE library of image, video, book, etc data. (It is a "cute" idea to be able to start an edit on your iPhone on the plane home and finish on your MacBook in your easy chair. ) I can hardly recall last month(over 70 guy) and I don't often care to show my neighbor a funny picture of his dog I recalled on the spur of the moment from 2 yrs ago. Certainly, others with a real memory will disagree! It all boils down to me storing on a hard drive 100% of my data and almost never accessing it. Kind of the same way most folks open all those old 3" thick musty photo albums in the attic.
If I can have 15% of the recent data in hand, I am good to go.