There are two ends here, as a user and as a developer. As a user Docker images just work, so you solve almost every problem you’re having which would be your users having them and giving up on using your software.
Then as a developer docker can get complicated, because you need to build a “system” from scratch to run your program. If you’re using an unstable 3d party package or missing packages it means that those problems would be happening in the deploy servers instead of your local machines, and each server would have its own set of problems due to which packages they didn’t have or had the wrong version, and in fixing that for your service you might be breaking other service already running there.
It’s not. Imagine Immich required library X to be at Y version, but another service on the server requires it to be at Z version. That will be a PitA to maintain, not to mention that getting a service to run at all can be difficult due to a multitude of reasons in which your system is different from the one where it was developed so it might just not work because it makes certain assumptions about where certain stuff will be or what APIs are available.
Docker eliminates all of those issues because it’s a reproducible environment, so if it runs on one system it runs on another. There’s a lot of value in that, and I’m not sure which resource you think is being wasted, but docker is almost seamless without not much overhead, where you won’t feel it even on a raspberry pi zero.