it was really for cross-distro GUI desktop applications. I saw it's used in embedded linux projects that has no GUI, for its portability at a heavy price(flatpak is quite fat, it needs to install a full sandbox)

I chose flatpak some time ago over snaps for gui apps and I don't remember why. I think there are benefits to packaging software this way (especially for immutable OS images) but at the same time there are so many negatives too. I hope they make it more of a priority. Or something better comes out on top.