It may be of interest for attackers that have no visibility into your network and make cross site attacks against you easier. For example, if I know that your router is available at router.network.internal, I might just try and see if your browser is logged in and send you a link to a page that starts making requests against that interface. Enumerating network resources can certainly be done via other ways, but DNS is a particularly easy one.

Interesting but 192.168.0.1 or 192.168.1.1 would probably cover 80% of that use case.

This is just security by obscurity. It doesn't add much if you already implement actual security in the form of firewalls, network segregation, etc.

I agree that it's certainly not a substitute for a proper security setup, but it definitely makes an attackers life easier - up to the point where it may get interesting to automate this. So as always, it's a tradeoff. Both options may be acceptable (and I've run a setup where I'd have internal names exposed on public DNS for years), but you need to be aware of the tradeoff to make that judgement.

I guess it could be useful for human level attacks, something like

"Hi it's Bob down in IT, any chance you could reboot 'funkyserver123', it's on 172.18.12.5, for some reason I can't get in"

Why would I answer a request from Bob down in IT about my home network?

Edit: I don't know why this is being downvoted. The OP is about a home k8s setup, so unless you live in some castle or palace I very much doubt there will be a Bob down in IT.

Possibly useful for SSRF attack but the matter is SSRF.