I use them pretty extensively. That way, whenever I need to rotate credit cards for whatever reason, I only need to update them in one location, rather than several.

Also, I’m using a single, common storage of credit card information, rather than needing to track 100s of different websites with potentially even more lax security.

I only use it when a seller doesn't offer any other way to pay, but there are still many, many sites that have it as the only option. A major one is Discogs; also quite a few artists on Bandcamp.