A negative captcha has the exact same purpose as your run-of-the-mill image captcha: To keep bots from submitting forms. Image ("positive") captchas do this by implementing a step which only humans can do, but bots cannot: read jumbled characters from an image. But this is bad. It creates usability problems, it hurts conversion rates, and it confuses the shit out of lots of people. Why not do it the other way around? Negative captchas create a form that has tasks that only bots can perform, but humans cannot. This has the exact same effect, with (anecdotally) a much lower false positive identification rate when compared with positive captchas. All of this comes without making humans go through any extra trouble to submit the form. It really is win-win.