Training ML models has been the thing for CAPTCHA services ever since Google bought ReCaptca in 2009, and was originally used to provide training data for OCR, but then switched to other data. https://en.wikipedia.org/wiki/ReCAPTCHA

And even though you can do a lot unsupervised these days, supervised labeled data is still something really useful for training ML models (often in combination with larger unsupervised corpuses).