I've never used it -- what do you like about it?

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

I use boring Debian, so apt and older packages, and flatpak for a few programs that I want up to date.

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.

Why do you need 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.

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.

ensures software support when the developer in question is a moron

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).