I still think it is nonsense. Every attacking move by a team is unique, and so is every defensive set up. A shot from 15 yards out can't have a definitive xG. It will be influenced by how many defenders are in the box, has the shot been taken by a recognised striker or a full back, was it on their stronger or weaker foot and so on and so on. There are so many variables so just to allocate an xG number is meaningless in my opinion (and I am never wrong).
Of course it's a nonsense, it's maths :)
All the match stats are, in a way, meaningless, even the actual individual results to some extent.
Shots on target, corners won, percentage possession, distance covered; all totally irrelevant and fundamentally meaningless. That's not the match. The match was the 90mins of football played between 22 blokes and officiated by some prick with his head up his arse. They're all different and none of them can really be actually predicted to any degree of certainty by probabilities. But like I said yesterday, it's just a shorthand way of expressing the match 'data'. A quick overview of the key important events, crunched down into number form.
Anyway, xG...
It's not meant to be some all conquering definitive answer to anything, it's just a simplification, and it's flawed because of this simplification, just like all stats are flawed (because their ENTIRE purpose is to simplify a situation), but the point of them is to show a condensed version of the entire game.
Obviously if the entire picture is needed then you watch the entire match, live as it happens, you can't get better than that. If you want an overview you have many options..
1) Watch the highlights - however this is going to be flawed, you only get to see chosen events, you don't see the panoramic picture just the camera view, you don't see how the match progressed...etc...etc...etc.
25mins? Maybe only about 10?
But you get an overview-ish.
2) Read a report - again flawed. Often opinion laden, often biased, only covers the events deemed important by the reporter...etc...etc...etc.
15mins? 10mins?
3) Have a quick glance at some match stats - flawed by their very nature for all the reasons you point out; and many, many more.
25seconds? 1min?
That's all it is though. Just a match stat. Just a more detailed version of total shot/shots on target which is designed to be consumed quickly and then move on.
Maths.