How does your convoluted Ansible system know which systems and services to maintain.

If its a list of IP addresses, having a list of ip addresses is a crude service discovery protocol.

Tasking developers (because lets be absolutely clear, the idea of removing DNS from production environments is something only a developer could come up with, no competent engineer would ever raise) with maintaining ordered lists of servers to keep updated is only going to overcomplicate things.

And yes your hosts file is another example of a list.

What you're asking is akin to "why do people sometimes need a boat to get from point A to point B? All I've used are cars and I think that should be fine."

You probably don't need service discovery if your entire infrastructure is small enough that the whole thing is deployed with Ansible.