- Akamai DNS
- AWS Route 53
- Azure DNS
- Cloudflare (excluding personal/hobbyist plan)
- Google Cloud DNS
And many, many others. And I note the site you posted this comment on is using Route 53, so probably paid as I doubt their query volume would be in the free tier.
Paying for DNS for personal/hobby stuff is probably pretty uncommon, because like you say, most domain registrars will offer it for free. But commercial websites often will, particularly larger ones with serious traffic.
Otherwise, that is my observation also.
A good provider will have different locations across the world, and users connect to the nearest datacentre. The free DNS some domain registrars offer is, sometimes, hosted at one single location. If the server is in the US and the user is in Europe, you're adding 80-150ms to requests. If they use "anycast" servers, the user could connect to a server 1-20ms away.