A cliche is an expression, word, idea, or anything else that has been used ad nauseum to the point of being so commonplace that it is practically useless.
From his definition alone, you can immediately begin to see why the use of cliches are not very helpful when it comes to writing essays. Cliches only fill space and are meaningless. Moreover, if you use many cliches, your essay will become annoying, and it will certainly put off your reader from enjoying or appreciating anything you write.
Effective writing is original. Even if you are only confirming what is known, you can do it in a way that is fresh. In short, stay away from cliches.