I was answering the "why do so many places use recaptcha?" question, not "what it captcha?", so my answer was correct as far as it goes.
In much the same way that spammers can use a spider to search for email addresses in text and send a spam to them; so can they spider for vulnerable web forms. People who have websites have at least some money and they're probably interested in my anti-baldness miracle food viagra pill, right?
So website owners put captcha on as a defence against robots...it has nothing to do with user security and everything to do with stopping the owner of the website getting bombarded with spam. There are other solutions, but the captcha scripts are particularly easy to implement. So that's why.
I never use google's captcha myself and resent having to do free work for google in order to access my stuff. I have seriously thought about invoicing them for it; especially for the fuckers that reload 5 times so you're there for 10 minutes picking out fucking traffic lights.