Why would someone make their blog private?

Private blogs are usually for sensitive or personal content. Anything potentially embarrassing, sensitive, or personally identifying is considered private information. Content you wouldn't want your family members, bullies, or strangers to accidentally stumble across should be kept private. This is to prevent the wrong people from seeing it and using it for malicious/harmful purposes. Once something is posted publicly online, it will stay forever. Even if you delete it, there's a very good chance someone has seen it or even saved a screenshot of it.