Well, Docker is usually not needed.

I make self-hosted software and haven't yet created a Docker image for it, but some people do want it. I also think it's useful, especially if you want to run multiple apps on the same server while having each app mostly isolated for security/privacy/performance reasons.

I think a good self-hosted app should provide numerous ways of installing it (Docker, bash install script, tutorial for manual installation, etc.).

To some degree probably, but there’s also a bunch of stuff that just didn’t exist in the same way say 20 years ago when the web was newer.

Torrenting was around but now I have a pretty well developed set of stuff that runs on my home server for downloading it and getting it into plex.

And then there’s stuff that’s always been a server thing, like exposing a printer on the network or hosting a file server, DNS (now with adblocking)

What I’m trying to point out is that following this trend, soon exposing a printer would be a docker container

Certainly some of these work fine to run as on-demand programs instead of services for many use cases (e.g. jekyll, rclone). I would say there are a wide variety of use cases that could have some of these running as a service on a laptop or workstation when it happens to be up, but doesn't need to be particularly high availability, too.