To keep machines/bots/droids/scripts/Muslims/fatties from gaining access to content that has some proprietary value to a website, developers use a test that humans should be able to complete easily but machines would not. These are typically called CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) which, OMFG it's a Turing test.
Which leads me to this... what the fucking what?
Now I'm no robot, but what is that string?
I'm sure there's a fine line between aggravating users and defending against robots, but for fuck's sake, I'm just posting something to my own Facebook feed.