He gets to exercise power based on his valuation, and in that sense, it is real. He is now known as the world's richest person and first one point five trillionaire, even if he doesn't have one point five trillion dollar bills in his closet. He gets people to suck up to him for fractions of it - "do X for me and I'll give you shares worth a million"
If he dumped all of his shares the value of them would essentially go away, like with any commodity.