do { } while(0) vs. if (1) { } in macros [closed]

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Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?

When one needs to execute multiple statements within preprocessor macro, it's usually written like

#define X(a) do { f1(a); f2(a); } while(0)
 

so when this macro is used inside expressions like:

if (...)
     X(a);
 

it would not be messed up.

The question is: wherever I've seen such expression, it's always do { ... } while(0);. Is there any reason to prefer such notion over (in my opinion more clear one) if (1) { ... }? Or am I wrong in my observations and they are equally popular?

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