Power
Restaurant critics, whether they're one of the twenty people in the U.S. who still have this job or they're simple Yelpers, have the power to drive business to or away from an eatery. If one reputable critic or several hundred Internet wannabes say the barbecue at Bob's Beautiful Ribs doesn't really qualify as barbecue, just overpriced meat with sauce on it, then you aren't going to waste your money at Bob's, are you?
This can be both good and bad for restaurants. On the one hand, a savvy restaurant owner will stay tuned into restaurant reviews and respond to legitimate complaints quickly, thus saving his or her business and its reputation. On the other hand, a clueless restaurant owner will end up wondering where all the customers are, and might ultimately have to close if there isn't enough revenue coming in.