I observed a referee a couple of years ago. It was a premier team against a division 2 team, and the score was 11-1, he'd had a very good game to that point. I was stood with one of the assistants, a vastly experienced Football League official, and we were both surprised that he was adding time, partly because of the score but also this league is on a 3G pitch with back to back games and the normal practice is to not add much or even any time unless there is a major stoppage.
A player from the losing team, bizarrely, tried to hold the ball in the corner after 5 minutes of stoppage time had passed. Myself and the assistant both found ourselves saying "find a foul, find a foul", he didn't and a notorious hothead from the winning team came straight through the back of him resulting in a major fight. It was all so avoidable, and unfortunately he didn't get the guilty parties so his decision to play unnecessary stoppage time cost him several marks.
I don't buy that you have to play added time. Yes, law says you should but it also says you should penalise a keeper who holds the ball for more than 6 seconds, penalise even the slightest penalty encroachment, both things that hardly any referee does. Law doesn't support finding a foul in the scenario I gave above but many of us do. I would also say that the referee in the Man City game will be fully supported for PGMOL, as will Andre Marriner who did the same in putting Tranmere out of their misery. Sometimes to be a good referee you need to know more than the laws.