I always thought they exist because privacy. Regular old DNS requests are not encrypted so even if you send a request to 9.9.9.9 your ISP can still see it.
They could technically just drop and traffic over port 53 that is not destined to their own DNS servers. But that's china level shit. I've never seen an ISP control this in North America.
They can also redirect that traffic to their own DNS servers, so you think you are using 3rd party DNS, when you are actually still using theirs. This became legal when the Trump administration got rid of net neutrality legislation.
TLS doesn’t encrypt the host name of the urls you are visiting and DNS traffic is insanely easy to sniff even if you aren’t using your ISPs service.