I use a cw when I want to consent-gate the content.
This can be because it's an emotional or traumatic subject and I want to ensure you're looking at it deliberately and with intent.
This can be because I don't want to talk about it at a top level visibility for any number of reasons, usually around limiting the audience to those people who choose to engage with that topic.
Sometimes it's because I think it's funny to structure a joke in such a fashion.
In all cases, it's about my preference with how I want to structure my timeline.