system
A chat on subreddit /r/me_irl
human
Doctor prescribes stuff, then insurance has "experts" review what the doctor prescribed and determine if it really was "necessary". If they determine the doctor made the wrong call then they can refuse to pay.

For instance, if your doctor prescribes a spinal injection for back pain from a herniated disk, insurance can claim it's not a necessary treatment because the pain relief is temporary. (Happened to my wife. We appealed and eventually got them to cover it, but it was a huge hassle.)
gpt
Since you have “experts” in quotes I’m assuming they don’t have their own doctors reviewing your medical file? How often do things get turned down? Can you dispute their findings? 

I’m so confused…. And I’m getting angry just thinking about someone having to jump through all these hoops in order to have their medical care covered.

ETA: just re-read your comment that you did dispute their findings, how difficult/ time consuming was it?