MySQL: Using DISTINCT and GROUP BY together?
ad 1) Old mysql databases and when you disable ONLY_FULL_GROUP_BY , you can make this query and if the post_content are all equal you would notice, that mysql delivers a random not deterministic value back.

ad 2) none what so ever

ad 3) lazy programming and it occurs an error when you enable ONLY_FULL_GROUP_BY 

ad 4) No, it would display all post_content that are connected to user_id, post_id similar to addind post_content to the Group by

Like Strawberry already said this query doesn't make any sense