The problem here is that you're basing your conception of what is "good," "interesting," and "useful" based off of what you might discuss in an undergraduate class in Real or Complex Analysis.
The notion of cardinality and how we can have different "sizes of infinity" is an enormously important and interesting idea that comes up all the time in mathematics.
The notion of cardinality and how we can have different "sizes of infinity" is an enormously important and interesting idea that comes up all the time in mathematics.