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
Now would be a good time for them to showcase books on fascism and persecution of LGBTQ people in Nazi Germany.