The purpose of CAPTCHA?
The basic purpose of CAPTCHA is to distinguish between a machine and a human being. As a machine or robot can be used for filling forms with random data but a image or audio to be entered cannot be done by a robot or machine and hence a human only can do it.
The step that makes you prove that you are a human is basically a CAPTCHA(Completely Automated Public Turing test to tell Computers and Humans Apart). Initially, a CAPTCHA consisted of image or audio for verification. But there has been a recent change in CAPTCHA verification vector which just requires a user to click on a checkbox to verify himself/herself as a human being.
You can read a recent answer of mine to have a better understanding of CAPTCHA: Ankit Shankar Giri's answer to Why the authentication sign in needs to prove the user is not a robot?
I have answered a similar query and I think that you will be interested to read it :