AppImage has a feature that neither Snaps nor Flatpak do: You don't need a repository or special manager for them to work. You can literally just take the AppImage file to (almost) any Linux Desktop on a thumb drive and run it.
Provided that the application inside the AppImage is built in a portable way, for which AppImage provided zero support whatsoever, last time I checked.
FlatPak has Runtimes/SDKs for this, Snap has the one true Canonical-controlled base snap.
