Why only define a macro if it's not already defined?

This allows you to override the macros when you're compiling:

gcc -DMACRONAME=value

The definitions in the header file are used as defaults.

As I said in the comment, imagine this situation:

foo.h

#define FOO  4

defs.h

#ifndef FOO
#define FOO 6
#endif

#ifndef BAR
#define BAR 4
#endif

bar.c

#include "foo.h"
#include "defs.h"

#include <stdio.h>

int main(void)
{
    printf("%d%d", FOO, BAR);
    return 0;
}

Will print 44.

However, if the conditional ifndef was not there, the result would be compilation warnings of MACRO redefinition and it will print 64.

$ gcc -o bar bar.c
In file included from bar.c:2:0:
defs.h:1:0: warning: "FOO" redefined [enabled by default]
 #define FOO 6
 ^
In file included from bar.c:1:0:
foo.h:1:0: note: this is the location of the previous definition
 #define FOO 4
 ^