##Example:
Let's suppose that user 9999999 posts an answer (that's not really an answer, but a well disguised "Me too bro, me too."). If the answer is short (like a one, two or three liner), we can usually spot it right away and deal with. However, if it is one of those two-page spreads with a feature article and lots of pictures (you get the point - this is metaphorical here)... we will only see a summary. The lack of a comment from the flagger(s) leaves us to have to read (sometimes) the entire thing before we can confirm the flag. Beyond that, it sometimes may seem to the poster that they did something perfectly fine, since their answer has been sitting for an hour and not been removed (they won't see flags on their own post).
In such cases... your comment goes a long way.