The primary advantage to me is clear separation between application data which I assume can be downloaded again (container image), and my locally generated user data which I need to backup myself (container volume).

The other one is forcing the developer to bundle all of the dependencies with the application. It does waste disk space, bandwidth etc. but they are all cheaper than my troubleshooting time and comfort. If and when I need to change some part of the image, I fork and modify it as I would with any other software.

Speaking of sustainability, if the process to update is too difficult and time consuming, the human factor (me) will delay or fail it over long periods of time.

I am not saying that everything should be run in a container, but I will disagree that installing manually is always better either. Not every application is a main character that deserves this work from me, and not every person is a full-time administrator. Docker is just another tool in the toolbox, that happens to offer an attractive compromise for some of my work, but I'd also be happy to be convinced otherwise (as I was until recently).