Why do people wear nightgowns?

This is a very good question and dates back to when people lived in condition very different from today. There was a time when people did not have central heat and air and houses were very drafty. Double pane Windows did not exist. This led to people sleeping in bed clothing. Men and women both wore night gowns. Later pajamas came into fashion and most men opted to go that route while women wear gowns to day for warmth as well as enticement of the opposite sex.