Some nightclubs card everyone. It's not just to check age. It's a way to at least see the names of everyone coming into the club and for the doormen to interact with people and see how calm/drunk they are and exert some control over them and see how they act. If someone is aked for ID and gets agitated and starts in with the "why do I have to show ID, this is BS" response, then they are not getting into the club. If they drop the ID, fumble with it, etc., the doormen might take a cloer look at whether they are drunk.