Back in Docker's day it was (basically) the only thing out there that let you reliably package up an application in a portable way from any Linux distribution to any Linux distribution - before Docker it was prohibitively difficult to build your application on your Ubuntu desktop and run it on your CentOS servers because you had to wrangle system packaging, library differences, and see the app run in the same way as it would in production. Docker turned the runtime into one consistent thing everywhere (and the ability to ship it to the deployment target).
That's less novel in 2023 but solved serious problems a decade ago. In 2023 reproducible builds are important as reflected by efforts like Debian's reproducible builds or SALSA and nix zooms way beyond that to solve downstream problems, too
Speaking as a linux user since 2004: it absolutely was not "prohibitively difficult" as shown by every application pre-docker. Beyond that, it's hardly dazzling
It lets me run up-to-date daemons on my home server, with excellent uptime, on some old-ass version of Debian I never bother to update (don't worry, it's not routable from the public Internet) and without having to touch systemd (or sysv init, or openrc, or whatever), having written nothing but a single short shell script for each service. I could take all my exact same knowledge and scripts (and a backup of the data directories, helpfully and completely documented in those same shell scripts) and spin my whole stack up again on basically any other distro, only having to google what installing Docker looks like for that system.
Different init system? Older/newer packages than I want in the distro's official repos? Some futzing-about with third party repos needed to add to get what I want at a version that's not three years old? No option but downloading a tarball and dicking around with that, on this distro? Package on this distro stores some daemon's config in a different places from the last one, and carves it up differently, so you can't just drop in your old config and have it work? This distro or version puts data for this daemon somewhere different, so now your backup scripts are fucked?
I don't have to care at all. About any of that.
I no longer have to give any shits which distro or version my home server's on (within reason) and anything I learn managing it transfers basically anywhere.
I can turn around and immediately apply nearly 100% of that skillset to any day-job I've had in the last decade or so.
