In 2012, Google started using captchas to identify house numbers from google street view. [2]
Now, users are identifying cars, bikes, traffic lights, and crosswalks most of the time. While Google/Alphabet has been mum on what specifically they're using the data for, it is speculated by engineers at competing firms that they are using this data to help Alphabet's subsidiary, Waymo, with its self-driving car program. [3] This data is either used as training data or to validate outputs that were already classified by their system.
[1] https://en.wikipedia.org/wiki/ReCAPTCHA
[2] https://techcrunch.com/2012/03/29/google-now-using-recaptcha...
[3] https://www.ceros.com/originals/recaptcha-waymo-future-of-se...