If you are going to be running an Atomic/immutable distro, you really want to use things like flatpack/snap/appImage to keep your user space separate from the OS.
Oh, you can sledgehammer an rpm/deb/what ever into the underlying OS. But if you do that, why did you choose an immutable distro in the first place? It's kind of the whole point.
Flatpak is a common way to install something newer than you can get in your repo. If you are using apt in Debian Stable, Flatpak is a miracle. This is even the reason Ubuntu installs Firefox as a snap (their version of Flatpak).
Not OP, but I like Flatpak (in addition to Apt) because it doesn't require escalation to add or remove packages, so my kids can self-serve adding or removing games.
Because it just works. After being with computers all day fixing the insane problems that other people create I just want to come home and press buttons and have things work
When using certain apps I prefer them being containerized on my system. It's case-by-case for me. I keep steam containerized, my web browser containerized, etc.
In the case of steam and web browser, the containerization means I can control their access permissions via flatseal. This adds another layer of security, since they're both web-accessing applications, and it's easier than setting up a VM to run those applications.