@yeifeiyu2 I asked the same question to the admissions office. They said the cover letter technique is AN approach but I didn't inquire about the "context" of the essay of whether it should be pre- or post-SIPA. They also said that it should logically tie-in with your SOP and resume (I think they are hinting at addressing the employer/position you talk about in your SOP).
If the intent of the application is to learn every aspect of the candidate, I assume we should talk about or maybe expand on areas not adequately addressed in the rest of the application (SOP, Resume, Quant/Lang Resume, etc.).
When they reply to you, please share!