Yes for most domains encapsulation is fine. It is not fine, if the domain is the device itself. I want to apt update the phone, I want to switch to another kernel on boot, I want to update the firewall from the shell, I want to sync config by rsyncing parts of my home directory. All of these won't work by running an App on top of Google Android, because I want to change the layer below.
> ssh with Xorg
For example to run gufw on the Phone, but access it on my computer? Sure I could also learn to do it in the shell, but it is so damn convenient. Sometimes it's also nice to run an editor directly on the device itself and not run the editor locally on an ssh-mount.