Why one should use API's in place of system calls in the code?

Question

If I have changed the no of parameter at some place where the function is being called. How can I be assured that no place is remain. Because build is successful, I cant not declare in header file because it is already extern in many files.

Brief example is given below.

/* main.c */
#include <stdio.h>
extern fun (int i ,int j);
int main()
{
printf("\n In main \n");
fun(1,3);
return 0;
}
======================================================
/* fun.c */
#include<stdio.h>

int fun(int in)
{
int local1 = in;
int local2 = *(&in +1);
printf("\nlocal1 = %d\tlocal2 = %d\n",local1, local2);
return in;
}

Answer 1

One reason could be "API provides abstraction from lower level implementation" .