Why do you use the static function of C?

int f1(int);        /* prototype */
static int f2(int); /* prototype */

int f1(int foo) {
    return f2(foo); /* f1 and f2 are possible because they are in the same translation unit */
}

int f2(int foo) {
    return 42 + foo;
}

int f1(int); /* prototype */
int f2(int); /* prototype */

int main(void) {
    f1(10); /* f1 is seen on the linker - ok */
    f2(12); /* f2 is not visible to linker - error */
    return 0;
}