Nitro is pretty popular. I know a lot of people including myself that cancelled it over the age verification bs.
They still clearly make money through ads and at the very least they scan messages for spam prevention, but also the fact that they are able to guess users age based on discord usage implies they're profiling users. For which they probably also utilize message content.
You need to fill tax info and everything, which is a very good reason to not need ID because your tax number should be identifying already. I guess they want to combat fraud, but this still seems like a complete overreach.