I started looking into this category of HW - SBCs, that is - 4 years ago. Initially, it was mostly curiosity ; to see whether I could apply my UNIX knowledge on these tiny machines, and whether I could coax them into doing actually useful things.
The result: I am addicted :-)
Not in the sense of idiotic consumerism - no, I only bought two of them (a Raspberry PI 2 and an Orange PI Zero). But in the sense of realizing that they can be put to many, many uses - and perform this work utilizing only a fraction of the energy cost of even the best of x86 designs.
I wrote a detailed blog post [1] about what I did with my 11 Euro (not a typo) Orange PI Zero. After that, I set it up as my NFS and Borgbackup server (i.e. I attached my two external portable USB drives on it - and it has multiple USB root-level hubs, so it gives me decent speed on them). A complete backup of my Arch Linux laptop (250GB SSD, MB Air from 2012) takes around 6 minutes to complete over Wi-Fi - and due to the magic of Borgbackup's deduplication, I have about 150 such complete "images" in a bit over 300GB total storage on the two drives. A periodic "scrub" run once a week makes sure there's no bit-rot - and if there is, the btrfs mirror over the two drives corrects it.
As for my Raspberry PI2...
- setup as an `sslh` gateway; this means that I can use `sshuttle` from the outside world to create on-the-spot SSH-based "VPN"s and access places that are blocked (during travels, work, etc). It basically "transports" my laptop/work computer into my home LAN.
- At the same time it is serving up my collected notes over the years on a Let's Encrypt-ed HTTPS private site via NGINX.
- FreeDNS provides a nice, easily accessible name for both these services.
- An attached RPI camera is used to take a picture every 5 seconds - and if the picture is sufficiently different from the previous one, it encrypts it and uploads it to my Digital Ocean droplet. Whatever happens to my apartment while I'm gone, I get a record of it on a remote server that only I can decode (the remote server doesn't have the GPG key necessary to do so, I have those on me).
- The SPI functionality of the PI is also used in my various micro-controller experiments - e.g. to "flash" my ATMEGA328p and have it do silly things [2].
- In general, I am tinkering with electronics - and there's a lot of fun in doing this with SBCs.
- At some point, I had it run exim4 as well; and got my mail-tester.com score up to 10/10.
The PI2 was 35Euros - the Orange PI zero 11 Euros.
The fun I've had building the stuff above? Learning skills? Connecting to them over serial ports and reliving my Uni-days?
Priceless :-)