Online, short question-and-reaction phrases like what's that brother travel mainly through short-form video, memes and soundboards, where a single line can become shorthand for spotting something unusual. Creators reuse it because it is instantly understood without context: the listener hears curiosity and disbelief and knows to look for the odd detail. It sits comfortably alongside other brief reaction sounds, works in gaming, vlog and commentary edits, and stays popular because new strange moments keep giving it a fresh target.