I mean, in this case I can actually think of two reasons.
1) A lot of people, particularly people who would be in the mindset to install PostmarketOS, probably have an old Android phone lying around, or at least know someone who has a phone they'd give up for free because it's currently living in a desk drawer or something. This means you can get a home server for free. Android phones aren't exactly the fastest thing in the world, but they're probably fast enough for a lot of servery things, and even a relatively small 128GB internal disk can still stream a fair number of movies if they're generously compressed with H264 or something.
2) Power consumption. My phone charger that I'm using right now peaks at 20W, and idles at considerably less than that when the phone is fully charged. That's getting into "guilt-free" territory; you can leave this plugged in and not really have to worry about your bill spiking as a result.
The translation: “With the help of simple tools, a loaf of white (or rye) bread can be turned into a trolleybus…
But WHY?!”
The label on the wrench says: “Skillful hands.”