Captcha (Completely Automated Public Turing test to tell Computers and Humans Apart ) according to wikipedia is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a human being. Usually used to fill the word verification when you login or register (sing up) online forums or register for a new email.
Why do people have to type the captcha code it? Because each computer and a sophisticated program would be difficult to solve captcha codes with high accuracy.
Captcha for what purpose? Currently captcha used for positive purposes. To save important archives in the world.