What is the main point of #define?

Question

What is the main point of #define, if it is a constant that allows you to replace anything you want with "turn off" some lines of code if it does not fit for something, or create a short function, but why it is actively used in all libraries and how it is on affect them?

Answer 1

#ifndef GUARD_HEADERNAME_H
#define GUARD_HEADERNAME_H

//This header code can be included more than once.

#endif

Answer 2

//In old code, bad code and rarely good code
#define WILL_BE_REPLACED(args) code that uses args;

Answer 3

#define WINDOWS
#define NO_EXCEPTIONS

Answer 4

Because its permits to signal feautures (or feauture lacks) to compile-time when allow some custom automated compilation