Why are some concerts 21 and over?

A local music venue has all ages shows on some nights and 21+ shows on others. Having attended both types of shows, I’ve found that they serve alcohol on both nights – except the all ages shows have special stamps for people who are over 21. So if they have a system for making sure underage kids don’t get served alcohol, what is the purpose of having 21+ shows?

I ask this because of a band that came through that seems like it would have attracted a large high school crowd, except they decided to make it 21 and over.

What is the purpose of that? Is it some sort of strategy?