Because most websites are open to the public, they are vulnerable to abuse.
Man/machine tests are designed to protect a website from parasitic abuse from other websites. An example of "parasitic website abuse" is where website "X" has spend considerable effort to offer a service to it's users. Website "Z" plunders information from website "X", and re-badges it without permission.
The access code you see below is easily read by humans but not so easily by machines, thereby protecting information from 'bots while offering it without membership or password, to human users.