Because it’s based on fedora atomic it uses rpm-ostree, which lets you layer packages to persist between updates. Good for stuff that isn’t available as a flatpak or doesn’t work as well when installed as a flatpak. Beyond that not much, maybe if someone doesn’t trust Valve with their OS?

On SteamOS you can still kind of have non-flatpak packages persist by using distrobox. It’s still a sandbox, just one that has a bit more latitude in what you can install than a flatpak.

edit: distrobox seems more integrated with the host OS than I thought