If you could find out one single fact about every person you met, what fact would you want to know, and why?
The thing about the Mirror of Erised that was the Most Important Bit in recent time was its sole functioning magic -- namely, the ability to show a person their deepest, most intense desire. Pretty brilliant trick, if anyone should contemplate it for a tick. Pretty invasive, they'd suppose, if they thought about it any longer. Violating, almost. Not like It can choose whether it will or not, of course. That's not It's purpose, which was originally to restore life (if that life was the deepest, most intense desire of the person that approached it).
Recently, though, the Mirror was feeling neglected. And when neglect enters the picture, it's not the deepest, most intense desires of others that any thing with a mind's going to focus on, right? If everyone were honest and open in what they were truly like behind closed doors and faces, they'd admit to thinking about their own deepest, most intense desires. Which was precisely what the Mirror was doing.
It wasn't thinking about how the wizards in ages past had passed It seeing a larger, more successful wizard; seeing their dead wife; seeing a hill of chocolates. It was thinking about trying to get around that. It was thinking about trying to be less invasive -- or, at least, invading a different area of the human being. What good are the depths of the soul of a wizard to a Mirror? The heart of the power of a human is not necessarily their emotions, but is more centered in intricate weaving of their mind.
Where are the transmogrification spells stored? Where does a human keep their knowledge of animation? Where does a human keep their blood-magic, their dark magic, their power? Where does a human store their contacts and knowledge of other human beings?
Where does a human keep the knowledge to turn a Mirror into a Person?
Their mind.
The Mirror of Erised
Harry Potter