The advantage lies in the fact that the passed code is preparsed for you. Remember the traps with things like x*x vs (x)*(x) in the body of a C macro? None of that silliness here.
While that is just nice, the power becomes impressive when you consider that since you can easily examine what the parameter code does without error prone manual parsing. A failed assert($foo == $bar && $baz == $quux); can automatically return an error along the lines of "$foo == $bar succeeded, but $baz == $quux failed". Given limited complexity, the macro could also automatically generate errors like "$foo (value: 21) was expected to be in the range from $bar (value: 8), inclusive, to $baz (value: 16), exclusive" when you assert($bar <= $foo < $baz);. Since the intent of a piece of code is implicit in itself, writing comments violates the principle of once and only once, so a reduction of human written commentary to be manually kept in sync with the code manually is very desirable.
The same principles apply for an ok() macro used in unit testing.
Makeshifts last the longest.