Let me get the first question out of the way.
No, you do not need to submit an optional essay. By definition, “optional” means you have a choice. And an adcom would rather read fewer essays than more.
Having said that, optional essays can be used strategically to explain a weakness (real or perceived) or include additional positive information that wasn’t included in the other essays. Note that some schools (e.g., Columbia, Wharton and Duke) state specifically that they only want the former. If you ignore their instructions you risk having them think you can't follow directions.
When explaining weaknesses, don’t whine or make excuses. If you have a low GMAT score, for example, don’t tell them how difficult the test was or that you were so busy with work that you could not study properly. The test is difficult for everyone and everyone is busy. Stick to the facts and wherever possible point out your strengths that may compensate for your weaknesses. Also, be proactive. If you know you have a weakness, take action to improve and use the optional essay to let the adcom know what steps you are taking. On more than one occasion I’ve recommended clients take a university extension course in calculus, for example, to compensate for a low quantitative score on the GMAT. Impress an MBA adcom with your self-awareness and your commitment to improve yourself.
You can also use an optional essay to provide balance. Introducing something personal through an optional essay, for example, is a great way to diversify a portfolio of essays that is heavily weighted towards professional topics.