"Is there anything else you'd like the committee to know?"
Is there a right answer? Do I talk about diversity or how much I want to attend their school, or is it for D/F/W grades? What if I have a W in a class that's not required and it didn't affect my GPA?
There isn't an answer that is right for every applicant; however, there are certainly many, many wrong answers.
Only include things that you feel the adcoms need to know in order to fairly assess your application. Theoretically, anything that is that important really should have been included somewhere in the primary application (or in other parts of the secondary, which the schools would have asked for if they really cared about it). The decision to include something for an optional essay should meet a much higher bar than for required essays.
I would not talk about your W. No one will care. Writing about this would only show neuroticism.
Is there really nothing you can compelling you can add to your app if you respond to this section? Like taking the opportunity to talk about why you're really interested in the school, talking about something you didn't get to include in your W/A, or just generally add more detail to something that you think would benefit from more explanation? (genuine question)
There are definitely some things that are compelling enough to be included, but these usually make up the minority of responses. For example, it would be more than appropriate to write about truly extenuating circumstances such as severe personal or family illness(es) that resulted in a semester of W's, a leave of absence, or a year of objectively bad grades (and not just bad by SDN standards, e.g <3.8 ...). Not feeling well during finals week or having a tough grader do not count.
I personally don't find much purpose to the "why us" answers. We already know that an applicant is interested by virtue of their shelling out a hundred bucks to apply. It may be useful for schools that are often viewed as "safeties" in order to screen out those with no real intention of matriculating there. There have been some responses that talked about personal reasons for why they needed to stay in the area, but these reasons need to be compelling and unique to the school, and even then, do not boost an otherwise non-competitive application. In other words, those who got interviewed would have been offered an interview regardless of that information.
For things that couldn't be included in the W/A section, if they were from restrictions due to the character limit or 15 activity limit, I would argue that whatever activity was bumped probably wasn't that essential (or rather, was the 16th most important thing). Just my thoughts
