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.