Feedback is useful because it is the best evaluation form of an employee or individual. Not only in a work environment, but also in a personal enviroment, feedbacks can bring new perspectives, since we can't really change something we don't know it's wrong. So when someone takes the time to give you feedback you get to change what you've been doing wrong.
Also, feedbacks can also be used to emphasize something good that the person has been doing, and then the individual will keep on following the right track.