I understand and like the idea of a math captcha, but I don't understand why there's an "I don't know" option.

It's useful to me, because I forgot basically all the mathematical notations I learned in highschool, but is there any effect to posting with that answer? Are posts with it downgraded in some way? It just seems like a very weird and possibly insecure system, especially since the captcha text next to "I don't know" isn't obscured in any way; it's easy enough for an OCR to read.