I watched a Ted Talk a while ago talking about how filling out a Captcha helps in the effort to digitize books by Google and others.
Essentially the random characters in a Captcha are not random, each are a tiny part of a scanned book page that needs verification by humans because computers have trouble deciphering that bit with with OCR.
https://www.ted.com/talks/luis_von_a...ation#t-205693