You're forgetting that in C you can call a function whose source you don't have.
C supports binary distribution of code, which is quite common for (commercial) libraries.
You get a header that declares the API (all functions and data types) and the code in a .lib (or whatever your platform uses) file. This is typically the case for all of C's standard library; you don't always get the source to the compiler vendor's library but you must still be able to call the functions, of course.
For that to work, the C compiler must have the declarations when processing your code, so it can generate the proper arguments for the call, and of course deal with any return value correctly.
It's not enough to just rely on your source, since if you do
GRAPHICSAPI_SetColorRGB(1, 1, 1);
but the actual declaration is:
void GRAPHICSAPI_SetColorRGB(double red, double green, double blue);
the compiler cannot magically convert your int
arguments to double
if it doesn't have the prototype. Of course, having the prototype makes it possible to error-check that the call makes sense, which is very valuable.