I also am struggling to understand what secrets he would know that could be used.
Well he used to be a trade envoy, which I think involved various arms deals etc. He certainly had influence and the ability to make introductions, and perhaps any introduction he made would be trusted more than ordinary foreign parties. All people with dodgy histories they don’t want out in public are liable to be manipulated (explicitly with blackmail, or more subtlely by those in an inner sanctum who seem to be helpful) – Andrew apparently brought the spy to the palace without following the official protocols. That might be as much MI5 signalling to Andy that he’s not trusted to pick his friends as much as signalling to the Chinese that we know who you are.
Why would he need to be told any secrets in the first place?
presumably even if nothing else – he is aware of various royal movements and security arrangements, is likely (when he was a working royal) to have known when the PM was likely to be somewhere and also other visiting foreign heads of states.
I can’t come up with a reason the monarch needs to know any secrets let alone good brother
You don’t think Charles and Keir have discussed Syria, South Korea, Israel, Russia, or possibly Chinese involvement in damage to undersea cables in the Baltic? IF we were considering an escalation, or indeed a softening of our position you don’t think Charles would know before the BBC? That’s just the military type stuff. Plenty of environmental, industrial type issues that might be of interest to the King and also to random other parties.
