Right but why is a magic mirror that agrees with everything you say (including your darkest impulses) a good thing? What benefits are these other people getting

Right but why is a magic mirror that agrees with everything you say (including your darkest impulses) a good thing? What benefits are these other people getting from their mirrors? Should the magic mirror salesman have warned the king before he bought the queen the mirror? Does the fairy tale conceit make this discussion more confusing rather than clarifying?