I understand that if a source file need to reference functions from other file then it needs to include its header file, but I don't understand why the source file include its own header file. Content in header file is simply being copied and pasted into the source file as function declarations in per-processing time. For source file who include its own header file, such "declaration" doesn't seem necessary to me, in fact, project still compile and link no problem after remove the header from it's source file, so what's the reason for source file include its own header?
The main benefit is having the compiler verify consistency of your header and its implementation. You do it because it is convenient, not because it is required. It may definitely be possible to get the project to compile and run correctly without such inclusion, but it complicates maintenance of your project in the long run.
If your file does not include its own header, you can accidentally get in a situation when forward declaration of a function does not match the definition of the function - perhaps because you added or removed a parameter, and forgot to update the header. When this happens, the code relying on the function with mismatch would still compile, but the call would result in undefined behavior. It is much better to have the compiler catch this error, which happens automatically when your source file includes its own header.
The header file tells people what the source file can do.
So the source file for the header file needs to know its obligations. That is why it is included.
Practical example - assume the following files in a project:
/* foo.h */
#ifndef FOO_H
#define FOO_H
double foo( int x );
#endif
/* foo.c */
int foo( int x )
{
...
}
/* main.c */
#include "foo.h"
int main( void )
{
double x = foo( 1 );
...
}
Note that the declaration infoo.h does not match the definition in foo.c; the return types are different. main.c calls the foo function assuming it returns a double, according to the declaration in foo.h.
foo.c and main.c are compiled separately from each other. Since main.c calls foo as declared in foo.h, it compiles successfully. Since foo.c does not include foo.h, the compiler is not aware of the type mismatch between the declaration and definition, so it compiles successfully as well.
When you link the two object files together, the machine code for the function call won't match up with what the machine code for function definition expects. The function call expects a double value to be returned, but the function definition returns an int. This is a problem, especially if the two types aren't the same size. Best case scenario is you get a garbage result.
By including foo.h in foo.c, the compiler can catch this mismatch before you run your program.
And, as is pointed out in an earlier answer, if foo.h defines any types or constants used by foo.c, then you definitely need to include it.