Like Irene said, "at 9 sharp" adds the nuance that the clock MUST read "9:00". If the clock reads even "9:01", you are late. Or if it reads "8:55", then you are too early. When people say "at 9:00", people generally don't mean literally, so you could arrive at 8:55, 9:05 or something reasonable. 9:30 is not reasonable. Hahaha :)