it's a fun toy, not super useful but probably fun to tinker with
I've done some ungodly stuff to my android phones (even non-rooted ones, I'm totally abusing them) and I can't even imagine all the possibilities with a proper linux distro. Having a pocket pc with a full arm64 linux sounds awesome
I daily drive it, just like some other people here. It lets you run the same software that you use on desktop. Some apps don't have a UI that works well on mobile (possible workaround: play around with scaling) and some old ones might not have touch support (you would have to use those with mouse or keyboard), but often there are mobile friendly alternatives, so it's better to use those instead. It can't replace a proper PC, even if you plug it into a monitor and it's not a fast device, but it's usable and you can do fun stuff with it. You can run CLI programs and servers, run Kali Linux (NetHunter Pro), distro hop, make hardware addons (there are some exposed pins on the back) or simply use it as phone. You just have to be an advanced GNU/Linux user, because sometimes workarounds are required.
And when you add the keyboard addon, you can look like a true hackerman: