CAPTCHA has been used on many systems to prevent automated registration or process into a system. This technique was originally designed to prevent bot or spammers to do further actions that are easily be done by humans.
There are a lot of variations of CAPTCHA, but lately, their efectiveness has been dropped since there has been many research of CAPTCHA model and some researchers are able to break through the mechanisms by analyzing the image and they have a good system that could have accuracy above 90%. More over, there has been a lot of paid services on the Internet that sells these codes, so it's up to the site owners to think more whether this solution is still effective or not.
As users, we should also rethink about how CAPTCHA can be evolved in order to make bots still unable to go through, but in the meantime still makes the human comfortable with the challenges that are constructed against them. I believe many scientiest should came up with more bright ideas in the future