It doesn't do that, though. Humans also create fake accounts. It does make mass creation of fake accounts impractical, though.
I solved that in past by actually charging for my service. I think the internet would benefit from having more paid content and less ads driven stuff.
One thing that captchas do protect from is brute force attacks on user passwords. Although there are other possibilities (like making the connection slow after a number of attempts).
This is why you need a multi-layered solution to fight spam. Captchas reduce the amount of fake accounts, which can then be taken care of by the additional layers.
It's easier to fight against a bigger opponent (botnets, etc) if you mitigate their superiority in number first.
I solved that in past by actually charging for my service. I think the internet would benefit from having more paid content and less ads driven stuff.
One thing that captchas do protect from is brute force attacks on user passwords. Although there are other possibilities (like making the connection slow after a number of attempts).