Specifically, guarding against multiple inclusion of header files:

• Improves compilation time.

• Reduces the size of object files generated using the -g compiler command-line option, which can speed up link time.

• Avoids compilation errors that arise from including the same code multiple times.

For example:

/* foo.h */
#ifndef FOO_H
#define FOO_H 1
...
#endif
/* bar.c */
#ifndef FOO_H
#include "foo.h"
#endif