If you have servers in your intranet, but don’t have enough expertise to install a CA server and a CA certificate on end-users’ machines, maybe move to a cloud based solution instead of hosting your own.
I have a NAS, HomeAssistant, some random local-only IoT-type stuff, Plex, Pi-hole, and a handful of several other web applications running. Running my own PKI and having to manually distribute root cert to my non-domain PC let alone even think about how to install root certs on the various Android/iOS stuff I have: no thanks.
I can think of several scenarios where a small business (with no IT) would have local servers but no dedicated IT (and where a full PKI infrastructure is a big burden): for example poor internet connectivity, or very high bandwidth costs.
You can choose to run Pi-holes, Plex, and all that crap in your intranet and pretend you don’t need a CA. It’s not that hard to set up.
Either learn how shit works or don’t do it.
To be clear, I’m still talking about a scenario where you are running a company and process data for your customers. Hobbies are different.
Not if your threat model is someone who already has access to the local network (which has no one managing it) snooping on traffic.