When I was in school this was called "new math." That will tell you how old I am. Anyway, I only learned some of the very basic stuff and never had a use for it since.

Wikipedia has this under "Applications"

Well, maybe so, but set theory was not even invented until late 19th century, after most of the other math had already been discovered.Applications

Set theory is seen as the foundation from which virtually all of mathematics can be derived. For example, structures in abstract algebra, such as groups, fields and rings, are sets closed under one or more operations.

Everybody learns this in school nowadays, so it must be pretty important. Educate me.