They're solving for everyone who needs captchas solved.

It's a very old service, active since 00s. Somewhat affiliated with cybercrime - much like a lot of "residential proxies" and "sink registration SMS" services that serve similar purposes. What they're doing isn't illegal, but they know not to ask questions.

They used to run entirely on human labor - third world is cheap. Now, they have a lot of AI tech in the mix - designed to beat specific popular captchas and simple generic captchas.