It puts the whole #define code into a single expression so you can do

  if (cond)
    MACRO;

and the macro won't expand incorrectly to

  if (cond)
    foo;
  bar;

I personally just go the anal retentive route and use brackets everywhere.

No syntax error if someone writes if(x)defer. Probably better to have a big error message since we can't curry the if.

if(x){defer} will be wrong, but it'll be wrong in a different way.