I have seen several questions are protected by Community or any moderator on main site. My question is, in which case Community or any moderator protect a question?
Questions are protected when a question keeps attracting low quality "answers" from new users. The Community user will do it automatically after the third downvoted answer post from a user with low reputation points has their answer deleted, or if it quickly attract five answers from low reputation users. The idea is to protect the site from "thanks" and "me too!" non-answers.
Moderators, and users with 15k reputation, have the discretion to do it at any time. They can also un-do Protection if it no longer seems necessary.
More here: What is a “protected” question?