I like them for the opposite reason. I'm still quite new to Linux, so I'm figuring out which software is best for me. I set up my server with Xubuntu and installed everything through Apt. I uninstalled a lot of software, but inevitably missed some things like libraries and config files.
Using Flatpak seems to keep track of everything, so uninstalling gets rid of everything that I would otherwise miss.
If it's doing what it says on the tin, Flatpak is making my life much easier :)