Question: why use 12 hour clock?
Answer: Why do we use the a.m.-p.m. system for telling time? ... Anyway, as near as I can tell, the 12-hour clock goes way back to ancient Mesopotamia and Egypt. The Egyptians used a 12-hour sundial to tell time during the daytime and a 12-hour water clock at night. The Romans also used a 12-hour clock.