C++宏什么时候有用？

#define HTML(NAME) "<" NAME ">";

#define ENABLE_DEBUG 1
#ifdef ENABLE_DEBUG
    #include<iostream>
#endif

#define IF(...) __VA_ARGS__
#define WHEN(...) IF(WHEN_, __VA_ARGS__)
#define WHEN_(_x, ...) __VA_ARGS__
#define WHEN_END IF(WHEN_END_, ...)
#define IIF(...) WHEN(IF, ...)
#define iif WHEN(IIF, ...)

#define CONCAT_(A, B) A##B
#define CONCAT(A, B) CONCAT_(A, B)
#define PRIMITIVE_CAT(A, B) CONCAT(A, B)