Let me just say that Flatpak (and snap + AppImage) hasn't even been on my radar until just now. But I think I just found a use case that I care about.
I like stable systems. I only run "LTS" releases. But there are a handful of programs where I do want the latest version. Usually I download the binary, install it by hand, make symlinks, .desktop files, and whatever integration I need by hand. This works. But Flatpak gives me a secondary package manager entirely in user space with bleeding edge software.
I think this an important use case. A rock-solid stable OS maintained by one package manager, with a handful of key apps at the bleeding edge, maintained by a secondary package manager.