A CAPTCHA is something that is designed to tell a human and robot apart. It does it by posing a problem which is easy for human but difficult for computers. For example, extracting text from randomly distorted image. Takes (a few seconds for) a dumb human to solve, but very sophisticated AI engine for the same problem, that too with limited accuracy.
But, computers are millions times faster to do rest of the boring work, for example filling up a form in a html page and submit it. Humans probably needs 30 seconds to fill one, where a automated bot can submit several 1000 per second.
Now you setup a service that’s human consumable, but open to abuse by bots. Most popular example is register a new user in your site (bots will do so in massive scale, use them to spam and abandon them). This abuse will probably deny legitimate users a service, and hence your service will be under DoS (denial of service) attack.
A CAPTCHA will keep such automated bots away (at the cost of adding a few seconds to human annoyance), and hence improve the security of your service (prevents DoS/flooding by bots).
