The signs and the ashtray are a belt and suspenders approach to mitigate a situation where a lit cigarettes starts a cabin fire. Humans don't follow signs with

The signs and the ashtray are a belt and suspenders approach to mitigate a situation where a lit cigarettes starts a cabin fire. Humans don't follow signs with 100% reliability, so the ash tray is a backup. They're on the plane for safety reasons, and required by law, even where smoking is also prohibited by law.