Many schools have become extremely restrictive on how many essays they allow you to submit.
The crushing numbers of applicants has forced schools to streamline the evaluation process and they simply do not have the staff or time to read 1000 word essays from everyone. Of course, whether you're applying there or not, you likely know that Harvard, as an extreme example, actually has no required essays as part of their application! They allow you to submit one essay if you like, but it's not technically a requirement for consideration as an MBA applicant. While we don't recommend you submit to HBS without leveraging the essay, this approach from schools highlights the fact that every word does indeed count.
Fortunately, other schools are more lenient when it comes to making your case for admission, and the Optional Essay is a good example of how schools want you to have an opportunity to make the best case possible.
But when should you use it? Many schools suggest it be used to discuss anything unusual in your history, such as an arrest record, an abnormal gap in your work history or why you did not get your boss to write you a recommendation. Certainly the most common use of this essay is to apologize for past sins as an undergrad in your GPA or a significantly lower GMAT score than average. This is admittedly an overused area for optional essays.
From a GPA/GMAT perspective, it typically makes sense to use the optional essay only if your combo GPA/GMAT would be viewed as a significant weakness.
Certainly if both are low, and the circumstances that resulted in a low GMAT and a low GPA are interesting or valid, you can explain it in a very succinct fashion. But if you went to a tough or highly ranked undergraduate school and had a GPA or equivalent of 3.0 or more, and a GMAT score that is well into the school's 80% range, you may consider making no apology in the optional essay. If you are still feeling a little uneasy about your metrics, instead of talking about them directly, simply highlight in other areas of the application things which address your ability to handle the b-school curriculum. Often, this is better than speaking directly to a score or GPA.
Beyond this, you should simply step back from your application and ask yourself if there is anything that jumps out as particularly disconcerting.
Are you applying with only a year's worth of professional experience, for example? Or did you change jobs five times in three years? Sometimes addressing such anomalies in a simple, straightforward way is the best use of an optional essay. Your mission is to inoculate any obvious concerns the admissions committees might have based on your core qualifications. Whatever you do, just don't use it as an additional effort to argue your case. If you simply build upon the arguments in the rest of your application, you may be viewed as inept.