It could be optional for many reasons:
1. Because it is one less thing for them to read for the auto admits/auto rejects.
2. Because it makes people on the fence about NU research it and potentially find something about the school they really love
3. It could be a test to see who wants to go there vs. people just throwing out apps to every school they get a waiver too fishing for acceptances
4. They are intrigued about what students find attractive about their school to better highlight those factors for next year’s recruitment.
5. They are sadistic people who enjoy making students beg for their supper.
No matter the reason, if an adcom says he wants something in an app, you should probably put that thing in the app.
This is pretty obvious...
Yeah. It just seems like they want their cake and to eat it too (and I'm not picking on NW here, it's every school).
The vast majority of law schools accept their students based on their numbers, regardless of how kick ass their reason is for going to that school.
The vast majority of 0Ls pick a school based on money and rank/prestige, not necessarily on any substantive feature of the school.
I have more confidence than ever (having gone through half a cycle) that no school treats this as a pure
numbers game, but I also have more confidence than ever that every
school treats this primarily
as a numbers game. Obviously I don't have the proof that they don't use these essays, but I would rejoin "Why even have the 'Why X?' essay if it rarely ever helps marginal candidates, and most people get in without or without respect to what the essays contain?"