One reason is to make the program run faster. When you have a function running, there's quite a lot of overhead. The function's variables have to be loaded onto the stack when the function is called. Then they have to be unloaded again when the function exits.

For a big, multi-line function, that overhead is acceptable. For a one-line function it wouldn't be, so you use a macro instead. Macros are created with #DEFINE (like named constants), so they are handled by the C preprocessor. Basically it's just a global edit; the macro call is replaced throughout by the line of code that it represents. Result: when the program runs, nothing has to be called, but you still have the convenience of typing in a name with some arguments rather than a line of complex code.

The circumference macro you're thinking of would look more like this:

And why would anyone do that? The answer, obviously, is because they're idiots.

Related:

What is the worst real-world macros/pre-processor abuse you've ever come across?

On the other hand, non-idiots use macros, too, e.g.

Can somebody show me an exams about how to write macros that are used for conditional compiling like Unix

I wanna learn writing them to implement the. Further in my software

Ps : must be macros that used for conditional compiling

I'm sorry, but I do not really understand.
At first you need to specify the language you use. (I assume now it is C or C++). But in C, C++ all the macros are used in the same way. So probably would be better to give us more details or show some examples what do you mean.

This is the fourth thread that you have opened about macro programming in the past few weeks. Macros for conditional compilation is not a significantly unique use case to begin a new thread without having shown that you have learned from the others, so I am merging this post into your most recent thread on the subject.

Please do not continue this behavior in the future, and again, please review the Site FAQ to which you have been previously referred for acceptable use of this forum.

I don't see much about C and C++ and nothing on the lines I am thinking of so here goes my maybe first answer to another question.
In C and C++ you can start a line with:
and every time the compiler files THIS it replaces it with THAT. The THAT part can be quite lengthy and can have variables such that you have a macro with arguments. That topic is broad and deep and I stop here.

However, I do wish to note: The Joint Strike Fighter (JSF) program writes code for aircraft in C++ and has published their programming guidelines. Its easy to find with a search. Those guidelines declare that macros are not allowed. They can make the code extremely difficult to understand. You may have to force the compiler to save the preprocessor output so you can see what those macros have done to your code.
So,..., if you are at the point where you need to ask about macros, then the best advice is: Don't use them.

Part 2
In Microsoft Excel, Word, Access, and others, you can write VBA (Visual Basic for Application) code to do things. There is specialized grammar/syntax for the various Office applications. Such code is frequently, sometime invariably, called a macro. It is not, but it is called that anyway. If you want to go further this will be enough to get you started.

Writing C/C++ Macros: Rules, Tricks and Hints http://www.ebyte.it/library/codesnip...CppMacros.html

It's actually a really good read.