The Monarch has no power to force an opinion but that is not the point. Having a safe and secret place for a PM to discuss difficult situations with somebody who has probably seen most of them before is immensely powerful.

I know some believe that all government should be open/public but I suspect in many cases that means that real discussions don't take place out of fear of offence or backlash from the public. If you were toying with the idea of e.g. privatising the NHS or selling off a load of gold for raising funds, you might want to discuss these privately before they end up as a Headline.