This is a CAPTCHA. Its purpose is to make parts of a site harder to script than other targets. It’s not trying to stop all possible bots as much as the least sophisticated (and therefore most numerous) ones. Like much of cybersecurity, the point is to make things harder so that hackers look instead for easier targets. It’s a cat and mouse game between the bots and the CAPTCHAs, and companies like google making the latter have the resources to always stay a step ahead.