An essay is an exploration of an idea which needs to be defined before it is developed. Because the material in an essay always relates to this central thesis, it is necessary for the writer to introduce that thesis and make the reader aware of its importance and relevance. The introduction is the place where the essay has to make a good impression, informing the reader what is to come and encouraging him or her to read further (but without rendering the succeeding paragraphs repetitious). If the introduction is tedious or fails to make the rest of the essay sound interesting, the reader will not wish to continue.
Of course, when you are writing a class assignment, you can assume it will be read no matter how bad the introduction. But your introduction serves the same purpose as it would if the reader were coming to it voluntarily. You must give the impression that your essay is worth reading.