Have you noticed that, on the net, it is harder and harder to make the difference between a human being and a computer ? The solution found some years ago the Captcha. The numerical identity is complex because you never know if you are in front of a computer or a human.
But it is like all locks one day someone find the key, and nothing is as before. For example Google's captcha was broken last march. Someone try out another idea with image and there is also audio captcha.
The captcha are used to avoid rogue computer to attack or pollute your site. They are there to allows a computer (your server) to differentiate a dumb robot from an human being. In the same time we are trying to create computers, robot... that looks like more and more human, we face the difficulty to differenciate them.
The captcha is the first attempt to make the distinction and will be followed by many other. I will not be surprised when replicants from Blade Runner needs to be arrested.