That that simply to provide something to charge you with? If you are acting suspicious, surely they have the power to question you anyway curfew law or no curfew law?
Simply being out late is not sufficient reason to be stopped or questioned for most people. It gives them a reason to stop and question people who are out late and may be up to no good. If you look young enough that you might be out after curfew, that's reason enough for them to stop and ask you for identification.
Obviously, if they see you doing something illegal, they're going to stop you. But simply "being suspicious" can involve a wide range of things, and curfew laws give the police a reason to, at the very least, ask for identification if you look like you might be underage.
In general, I think these laws are intended to deter gang activity and what people might refer to as "anti-social behavior" elsewhere (petty crime, graffiti, etc). As a result, I'm betting they are more heavily enforced in inner city areas rather than in suburbs or rural areas.
Even better is looking young and driving past curfew. Again this is the kind of thing officer will use for a PC stop. Sure you're 20 but the officer doesn't know than until you've been stopped and provided license, registration, and proof of insurance.