"Then why say it's purpose is to be sat on?" Because that's what it's best at. What it's design is optimized for. Other furniture can fill those roles much more effectively … "What if a leg breaks, then?" You'd just get a new one … "You're looking at the wrong side of the question, Lottie! A broken chair is still a chair. What does the chair 'afford' after it's broken and thrown away?"