simple example: the app that you want is outdated, is missconfigured on the distro’s package manager (e.g. OBS on arch missing wayland capture). If this app has a flatpak version, it’s likely it’s mainained by the same people who makes the app, thus they can make sure it works fine through flatpak, and since it’s distro independant it works everywhere. App images just bring all their dependencies with them, and snaps idk never used them…

I personally use KeePassXC (KeePassDX on android), it can have TOTP code generation for 2FA for any service. And since it’s a password manager, it’s secured by a master password.