What does snap do that flatpak doesn't?
Flatpak only focuses on Desktop Apps. For more information, see :https://merlijn.sebrechts.be/blog/2020-07-03-snap-vs-flatpak...
Another big difference is that the sandbox of Snap is much more granular. Snap uses AppArmor to mediate the _existing_ Linux api's. This allows you to give an app access to usb-serial devices but deny it access to a USB webcam, for example. Or to give an application access to joysticks and controllers, but deny it access to a webcam.
Both Snap and Flatpak support XDG Desktop Portals to give desktop applications secure access to features such as a webcam, but this is a new API, application need to be ported to the new API before they can use these features.
As a result, most snaps have much less permissions than their Flatpak counterpart because the Flatpak sandbox needs to be largely disabled for the applications to work.
Seems like Canonical is crammjng everything they can into Snap - free software, privacy & security be damned!
[0] https://flatpak.org/faq/#Can_Flatpak_be_used_on_servers_too_
If you do have it installed then yes, you would have dbus.
For a desktop user, honestly, very little.
For that there is ostree, which is already supported by at least one major embedded systems vendor (https://docs.windriver.com/bundle/Wind_River_Linux_Tutorial_...).
First of all, the fact that you can put a system service in a snap: The OpenPrinting project is working on creating a snap which contains the entire Linux printing stack. This is so users of older distribution releases can still use the latest printers.
Secondly, the Snap sandbox is a lot more granular than the Flatpak sandbox. Flatpak went for portability and Snap went for granularity. Snap uses AppArmor to mediate existing Linux kernel api's. As a example, you can give a Snap access to joysticks and controllers but deny it access to a USB webcam, for example.
XDG Desktop Portals should make it possible to safely use certain host features such as a webcam in a Flatpak, but this requires applications to be rewritten to use the portals api. In the current state, most Flatpaks disable large parts of the sandbox because the applications would otherwise not be able to run in it.