Try filling out the captcha wrong and see if it returns an error. 
If it does then, I would think, it is used to verify one is a human and not a bot. Right?
Because if a spammer already knows what the captcha value should be why would they need you to fill it out for them? 
Or if I am getting it wrong please explain how some captchas are used to proliferate spam...
and how one can tell the difference between the legit ones and the spammy ones.