Can someone explain how and why and when this would be used?

I use one of their "dynamic domain" as a stable address to point to my home computer : example.mooo.com instead of using the IP. They provide an endpoint for a cron to update this IP each X time.

When you want to host your own nameserver but to lazy to configure it on your colocated server. Yet you have requirements that require a nameserver.

I've been paying customer for years. Branded nameservers are great, the web ui is janky and dated but seriously robust.

Support is lacking as for a while afraid.org has had an SSL cert error, very annoying.

similarly, someone please explain why a free citizen would voluntarily use a Google DNS service that builds profiles and extensive activity records on its users