Why use Macros in C? [duplicate]

Answer 1

I think that biggest advantage comes when used as "setting file"

#define USE_FAST_AGORHITM
//#define USE_SLOW_ONE

And global "constants"

#define MAX_NUMBER_OF_USERS 100000

There are programs written mostly in macros (Check for instance Brian Gladman's AES implementation http://www.gladman.me.uk/cryptography_technology/index.php )