Most of the other comments focus on how and why theory research often leads to "practically useful" outcomes down the line, so I'll instead say that not all res

Most of the other comments focus on how and why theory research often leads to "practically useful" outcomes down the line, so I'll instead say that not all research has to be practical. Sometimes people do things just because they're interesting and fun. Why do people play board games, or video games, or sports? Why do people read fiction? Why do people have hobbies which strictly distract from productive work time? The answer to any of these questions is the same as the answer for "Why do people do theoretical research?": it's fun.