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

While it is certainly possible to do as you did, the standard practice is to put shared declarations into header files (.h), and definitions of functions and variables - implementation - into source files (.cpp).

As a convention, this helps make it clear where everything is, and makes a clear distinction between interface and implementation of your modules. It also means that you never have to check to see if a .cpp file is included in another, before adding something to it that could break if it was defined in several different units.