Why would I do this over a Synology DiskStation which allows me a hassle-free way to stream my music anywhere I have a internet connection... I use it daily in my car without a problem, going on my 3rd DSM and ~10 years of this setup.
For me, the reason is that Synology is not open source, and additionally because their NASes are overpriced for their size and capabilities. I have a 12 disk NAS that cost me a total of $500 (before drives), with no software licensing fees at all, and that's not even an especially cheap one. My earlier NAS was slower but only $120. Of course, it needs a deep 19" rack to mount, but that's actually an advantage for me, as I have a dedicated homelab rack to play with.
Dedicated NAS hardware is often pretty efficient. The Synology DS224+ (a 2-bay model) for example claims to idle at 4.41W when the drives are allowed to hibernate. That's within spitting distance of a Raspberry Pi, and a lot better than most people will be able to achieve with repurposed desktop/server hardware. Lots of desktops will idle at tens of watts.
If you keep your spinning rust drives spun up all the time, it's another 5ish watts per drive.
