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For some reason or other, you might want to allow access to certain posts only to certain people. You can use password-protection on Pages and Posts, but those need to be set individually. 
It’s far simpler to just write a Post, specify a category and all posts in that category are password protected. Or you might decide to protect a lot of old posts. 
Imagine a family blog where certain posts are only visible to people who know the birthday of your first child, or remember your pet’s name. 
Cat-Pass is not for super-sensitive information, it’s for casual use.

Cat-Pass

Cat-Pass lets you password-protect categories. Caution: deactivating the plugin will render your protected posts visible again.

Cat-Pass vs Simple CAPTCHA Alternative with Cloudflare Turnstile