Apart from the fact that it's custom hardware designed to run Linux, (you buy this for the same reason you buy System76 or Purism's other hardware, even though you could build your own machine or install Linux on hardware built for Windows) I would much rather have a phone built on Linux than a phone built on Android. Android really doesn't offer anything more on a fundamental level than Google-made ease-of-use layers and a development platform that I can really take or leave.
I run Arch Linux on all my devices with no Desktop Environment. The simpler the system the better. Running Android Framework along with a runtime, even though it's all open source, I'd rather as much of it be free software as possible. If I could run my phone like this I absolutely would but the Librem 5 is as good as it gets while still having a device that fits in my pocket that can make calls and text.