This is basically a place to upload your resume. If there are activities (that are important to you) that you couldn’t include in your common app, or you couldn’t fully detail in the space the common app allowed you, here would be a good place to put them.
@Yogowild the prompt seems to indicate that admissions is offering a place for further detailed information that is pertinent to the application. Another essay is unlikely to serve this purpose.
I don’t think that explaining your favorites is necessary or even wanted. If everyone did as you propose to do, they would have 29,000 extra documents to read explaining something they didn’t ask to explain.
It is not simply for a resume. In fact, most of the time, a resume is not needed, just distracts.
Nor is it for an additional essay, as if that somehow scores you extra points. You use it to explain something they need to know, that they couldn’t figure out from your app. Most kids never need to explain anything extra. This applies especially to the top schools, which expect to see your judgment.
@Yogowild you may certainly add whatever you feel is helpful to your application. I and the others above are simply offering a suggestion that an additional essay to read may be a less welcome addition to admissions. My son attached a resume that had a listing of additional awards and honors and sports accomplishments that did not fit within the regular application. This type of document is a quick scan for admissions to read. Another use of the attachment might be life situations that adversely affected grades or ability to volunteer etc. and which do require some further documentation.
The favorites section is short and intended to be so. Unless there is something highly controversial within your favorites, there is no need to give any type of explanation. In reality, the favorites section is probably the least important section of the entire application so extra detail in an essay is not needed.
I understand the desire to be as thorough as you can be (that quality is what may make you a viable candidate in the first place) but part of the exercise of the application process is the ability to zero in on the most important components effectively - offering a clear picture of who you are and what you bring to the table while adhering to the parameters of the application.