Kubernetes works on it’s own if all you use is in k8s.
I’ve yet to set foot in a bigger place with distributed teams and services that only use k8s and nothing else.
In these settings there’s a need to interconnect and monitor everything.
I’ve even at one time registered mainframe stuff in consul, as an external service, with a rather complex health checks in place.
This alone improved observability and reliability for a lot of the distributed consumers.