BM Precompiler?

I don't quite understand the define use here, bear with me =P.

so, if I put this:

define(PROFILING, 1)

If PROFILING
 ''This code will be compiled
else
 ''This code wont?
end if

Sorry i've just woke up, but it looks like it just defines a variable, and would compile everything anyways. Can you explain this in more detail?
Or is this just an example proving that it can be done with a bit more work?