Captcha is a small image containing a code(above) that is used to identify humans from machines. The code visible in the captcha image is supposed to makes sense only to humans.
Since these images can only make sense to humans, captchas are often placed at login screens in order to stop bots or machines from automatically logging in or sending bulk requests in one go (which may sometimes lead to denial of service and server crashes).
Here's a screenshot of Employee's Provident Fund Organization, India's login portal to access passbooks. A portal that employees all over India use to check on their provident fund: