When you're reading a book and the beginning is really boring and doesn't grab you and you don't have to read the book. Do you really have the incentive to finish it? The whole point of an essay is to have the reader hooked on what you are saying, interested and they want to finish it. If the introduction isn't strong the reader doesn't want to finish the essay. It's like first impression is last impression. If the first impression that a reader gets of your essay isn't an extremely positive one do you think the review of your entire essay is going to be positive?