#textwarning

this drives me nuts everytime i see it. can semoone explain to me about tumblr "#gifwarning"? that sometimes appears on the bottom of posts? what i'm stuck on is, it's always at the bottom of the post, so it gets seen after the dangerous gif in question. why would you put a warning about something in a place where you're sure the reader of the warning will have already encountered the thing it's warning about?? like a falling anvil labeled "watch out!!" from one of those cartoons