Or patents and public disclosure.

Ie, if you're in the process of developing patentable technology or writing a patent, anyone to whom you disclose details needs to be under NDA, otherwise your patent may be refused due to prior public disclosure.

Or so is my understanding anyways, I am not a lawyer.

Hell, even without specific trade secrets to protect, Facebook wants to control the narrative about their project and thus wants to discourage leaks.

I mean, man, Apple is legendary for how hard they work to avoid leaks, but I've never seen anyone around here accuse them of using NDAs and equivalent in order to cover up crimes.

a battle over this has developed in the web go-go days. Developers were purposefully asked to sign NDA not due to patents, but to control the applicants to rival startups. Meanwhile, certain developers were so valuable that they discovered they could make demands themselves.

You're talking about a scenario in which you're getting paid? So there's a clear benefit, which you understand and accept.