Computer science is for studying what computation means and how we can describe it, the questions we can frame in terms of computation, how they might be answered, and what that can tell us. Applied computer science and computer engineering use that theory to invent machines that use the principles of computation to do useful work. Software developers use those machines to build solutions for specific problems, sometimes using knowledge of computer science.

Theoretical computer science can inform the use and design of practical tools that apply it, same as any other field. Theory is for asking questions and looking for answers. Application is for understanding real-world implications of the answers. Practice is for putting them to work.

The issue in the US education system is that we have one degree we call "computer science" which teaches all three of those pieces, each of which insists it's the real piece, and the students, faculty, administration, and employers all have very different expectations about what it means to have the degree.