That way, you assert the focus of the thread is on the underlining issue, and frame it in a context people can understand and engage.
A warning of the type of content to expect from the post so that anyone who just doesn't want to view it or perhaps is in an environment where that stuff might not fly can not bother with it.
Once you have a certain amount of reach, it won't matter if it's a joke or not, the wrong people will take it as affirmation.