It solves a specific instance of the problem of needing packages from two distros, which is a common instance of that problem: wanting a stable base set of packages on your system with the ability to use some newer, up to date ones.

Nix is highly relevant because it solves that case: you can run Nix on any stable distribution like Debian, and the Nix package repository contains a very large amount of mostly up-to-date software, similar to the Arch+AUR repositories.

Another problem it solves more generally is getting access to a large repository of maintained packages from a base system that only has a small repository, even if those packages are up-to-date.

Both of these are reasons you might use Bedrock, so it doesn't seem off-topic.

The only reason you would want packages from different distro's is if they have a different version.

With nix you can install packages with different versions, you can also install old packages by going back in nixpkgs history and building the package.

Nix being source based means you can install any software package as long as you have access to it's source.

Ofcourse some of this is possible with other systems too but the nix ecosystem makes it far easier to build packages in different languages.

In short nix can treat the disease (missing different versions of packages in distros) instead of treating the symptoms (installing packages from different distros)