I've been getting comment spam already. Sorry for the extra step to comment.
What this does is to prevent automated systems from adding comments to your blog, since it takes a human being to read the word and pass this step. If you've ever received a comment that looked like an advertisement or a random link to an unrelated site, then you've encountered comment spam. A lot of this is done automatically by software which can't pass the word verification, so enabling this option is a good way to prevent many such unwanted comments.