I think realistically any software you'd want to install as a flatpak would otherwise only be available as a package for a specific version of Ubuntu. Flatpak gives devs a way to package proprietary or cutting edge software in a distro-agnostic way, which is a good selling point for them. It's also nicer than managing apt repos and ppas.

The extra space usage is annoying, but it's not that big a deal. My mint install with a few flatpaks is still smaller than my Windows install.