I haven't used distrobox, so take this with a grain of salt:

• reproducibility: if you copy my nix files, the flake.lock will ensure you have the exact same results as me

• declarative package management: you make a list of packages and the declarative nature forces you to keep it up to date at all times, that's how you install/remove them. On arch you need to -Syu the package and remember to add it to an installation script (I never did). This also allows for easier maintenance because you don't need to go through random dependencies to find an unused package you've installed (~100 packages on the list == almost 2000 packages installed). If there's a distrobox version of a Dockerfile, you can do the same but it will most likely have the same disadvantages.

• home-manager allows you to configure packages (usually not worth it though)

• no need to export packages, when you install them they're immediately available in your main distro

• combine these and you have an extremely simple setup to distrohop, or work on multiple devices. You can also for example break off certain packages in a separate module, and only install them on a certain machine.

• I'm guessing updates are easier, and if it breaks something, you can easily rollback to a previous generation. It will not only revert to the exact same packages you used previously, but it will also revert any package configs it controls. And on top of that it lets you pin a package to a specific version, and upgrade everything else.

• cross-platform: you can take your list and install the same packages on win/mac natively. They don't need to run linux in a vm like a container would

• less storage used?

• temporary package installs. For example I only needed arandr for 30 seconds to set up a new monitor, so I just nix shell nixpkgs#arandr and it created a shell with that package. When I was done I just closed the terminal, and didn't need to think about it anymore. The package was completely removed the next time I ran nix-collect-garbage.

• you need a package that's in no repository, but it has a flake with all of the compilation dependencies? You just cd into the repo, nix develop, and you've got a temporary environment with everything you need to start following the compilation instructions

But in case of the repository size, Debian and Arch (+ AUR) are extremely big aswell.

Arch has a lot of packages, but there are some that I had to install through aur which I don't like. Nixpkgs have so far had everything I needed, except for nixgl (although I couldn't get a few of them to work). Also, you can chose between "stable" and unstable repos. Arch doesn't have a frozen version with updates every ~6 weeks, and no other release based distro comes even close in either quantity or freshness.

That's what I can think of for a casual user. There are a lot more benefits for professionals to be honest, and I wouldn't suggest nix at all if home-manager didn't massively simplify the whole process. Getting to those few simple files from above was a massive pain, and it's made even worse by the official nix guide suggesting outdated methods, and most of the support threads being for nixos. With them, you can get going in like 10 minutes even if you don't know anything.