Why should I not include cpp files and instead use a header?

Question

So I finished my first C++ programming assignment and received my grade. But according to the grading, I lost marks for including cpp files instead of compiling and li

Answer 1

When you compile and link a program the compiler first compiles the individual cpp files and then they link (connect) them. The headers will never get compiled, unless included in a cpp file first.

Typically headers are declarations and cpp are implementation files. In the headers you define an interface for a class or function but you leave out how you actually implement the details. This way you don't have to recompile every cpp file if you make a change in one.