In general, a "Why X" essay can be a great opportunity for you to explain why you want to go to a specific school and to show a school that you have researched them. So if a school gives you an optional "Why X" prompt, and if you have something to say beyond "I'm applying to all the T14 schools" or something else vague and general that every other applicant might write, then go for it. But if you don't actually have anything specific or worthwhile to write about, optional essays can be more of a hindrance than a help. And I wouldn't submit a "Why X" essay to a school that hasn't offered that as an optional prompt.
So try not to let yourself get too tangled up in all the conflicting advice you can find out there! Trust what the admissions offices at the specific schools you are applying to are telling you: if they say it's optional, it truly is optional. So ask yourself if you really have anything interesting to say. If you think you have real, specific reasons as to why you want to go to that school and you can write an essay that will make you look like a stronger candidate, go for it. (And, again, for the T14s, these reasons have to go beyond "You are a highly ranked school." The T14s know they're highly ranked. They know everyone wants to go to them because they are highly ranked. But the T14s are also competitive with each other and wouldn't necessarily hate it if you can point out offerings that make them more unique among the other schools.) If you can't identify any specific reasons why you want to go to that school and you can only come up with a generic essay that will end up reflecting badly on you, skip it.
I've linked to some more resources below for you:
Forum post on "Why X" essays:
Blog posts on optional essays:
https://blog.powerscore.com/lsat/bid-15 ... nal-essay/
https://blog.powerscore.com/lsat/supple ... yes-or-no/
Optional essay examples:
http://www.powerscore.com/lsat/law-scho ... amples.pdf
Hope this helps! Good luck!