The earliest ones that asked you to type two words were also used to help digitize the Google Books library, for cases when the OCR wasn't quite able to match a word confidently. That's why you'd get a pair, one that was clearer but "jumbled" to be unreadable to computers, the other looked more like a fuzzy scan (or occasionally just a complete mess of misplaced ink), though both had a wavy effect applied to them. Only the clearer one was needed to pass the test, the other was just free training for their OCR process.

Around 2023 when a bunch of Lemmy instances got stood up after the reddit debarcle. Most instances were still open sign up at that point, then the Lemmy devs removed the (basic) captcha from the signup page.

That day we learnt that even the most basic captcha still stops a LOT of bots.