The main role of header file is it is used to share information among various files.To put it brief, if we have several functions say 4 functions named as f1, f2, f3, f4 placed in file say sample.c and if all the functions want to get accessed each other all must be placed in the same file sample.c.
In other words if there is a function say f5 placed in another file say example1.c and if the function f1 placed in sample.c wants to access the function f5 placed in example1.c it is not possible. For this what we can do is add the function definition in the main program from which the function is going to get called. But this will make the process tiring if the program has lot of functions because if there is a change in arguments in functions each time programmer has to search for the function definition file and has to make change in the function definition file as well as in the actual function. These will make the process tiring.
To avoid all this and to make the process simple what one can do is have a header file and place the function declarations in a header file say for instance if the header file is sample.h where function declarations area made the programmer ahs to include this header file in the source file where the functions are used as
In this way of there is any modification in function arguments only eth actual function and the definitions defined in the single place namely header file has to be changed.