How are header files important?

Answer 1

I am new in c++,but I have already been using them for modular programming, that is I write some ferquently used functions inside a header file and use the header file whenever i need those functions.

Answer 2

a header file is basically text that is to be copied places before compile. That's why it uses #(preprocessor command) to include a header.

I usually use
-function declarations
-structs
-classes(functions in .cpp)
-defines
-other includes
inside of my headers

but in the end just remember it's just pasted code.