A sarcasm tag indicates that the poster knows that at least half of the people reading the post are to damned ignorant and/or humorless to recognize sarcasm when they see it.
Often, they are the same ones who can not link/differentiate subject and direct object (let alone an indirect object) in a clearly written sentence.
A sarcasm tag is also used to show that the poster understands that some people are too ignorant to know that there really are people out there who believe the ridiculous things they post.
You certainly got that right ! Thanks !