Mainline Linux support means the device is going to be supported for a very long time. For Android phones you're always relying on the manufacturer - even with the likes of the FairPhone you're still beholden to qualcomm's willingness to provide updates.

You also get the same software running as you do on desktop Linux - just hook up a display and you've got a regular Linux desktop. You can print from the device itself, because of course it can just run CUPS. You can direct audio output from each application (including to the earpiece) because it's just running pulseaudio. Hook up a webcam, a Wii mote, once you have actual control of the hardware the possibilities are endless.

The desktop environment is nice, but android can do all the rest of it. I've never had problems with printing from my phone and webcams and gamepads work just fine over USB.

Just speaking for myself, as somebody who has preordered a Librem 5, but may not be representative at all of the wider community around it:

I want a pocketable Linux device I can take with me everywhere. That's it. I don't care how well it works as a phone. I want a fully-featured, honest-to-god GNU/Linux terminal in my pocket that I can use to do anything that I could do on a (massively underpowered) Linux laptop, especially on the command line (i.e. I don't even care too much about GUI apps except some basics like web and PDF display).

If it can someday replace my phone, that's a massive bonus of course, but realistically I don't think it will within the next decade, for my specific smartphone needs. But that's OK - I don't mind carrying alongside it an Android device that is my actual "smartphone".

You might be interested in things like the Pandora/Pyra, the Cosmo Communicator or the GPD Win.

Thank you :-) I own both a Win 2 (Windows for gaming) and a Pocket 2 (Linux, for study/coding) and am very happy with both of them - but neither is actually pocketable, which is why I'm looking towards the Librem 5 (and have also considered a Pinephone, and would be very tempted to get one if they come out with something closer to Librem 5 in terms of power)

Right now? Probably not a whole lot.

Eventually once the software is more refined, there are definitely some features worth adopting for. The kill-switches are a decent feature, and I would love to have something like that on every phone (especially the cam & mic one, since I don't tend to put tape over my phone cameras!).

For the Pinephone, the hope is that one day you'll be able to buy one (or some successor) and be able to load whatever distro you want on it. It's vastly easier to work with than installing a custom bootloader, and then flashing a new version of Android. So as far as that goes, there's a lot to be desired.

Unlike android, most GNU/Linux OSes are built so that you can hack around problems on your own pretty easily without having to completely rebuild the OS. Also since you're just running X11/Wayland you have a way larger selection of software available. Almost anything open source that runs on your Linux desktop will run on your phone (although often with less performance.)

Finally, you're not stuck with a PalmOS style mobile GUI and can use a normal overlapping window manager.

The efficiency of developing software on GNU/Linux.

(edit: rewording)

Roughly the same thing that makes running Ubuntu (or any Linux distro) on your computer a better option than running a Google-less version of ChromeOS.