One reason to repurpose desktops is that you get a full ATX Motherboard with SATA ports!

If you are doing a DIY NAS with HDDs then you want real SATA ports. Or a well supported PCI card with SATA Ports, which you cant sensibly connect to a Laptop or micro PC. Sure, you might be able to use Thunderbolt to reliably hook up an external PCI chassis, but then you might as well buy a NAS at that point or use a full tower case with an ATX mobo!

Using an older Gaming PC you already have is actually a very good option for TrueNAS or OMV.

I took an older 10th Gen Intel Gaming PC we had, sold the core i9 CPU, and replaced it with an i7-10700T I found used on eBay.

I'm finding this setup to be better for my needs than various ex-lease Dell Micro PCs I've used in the past, mainly because of the reliability of the SATA ports.

I've found quality external Samsung T5 SSDs to be very reliable over USB with TrueNAS. But HDDs are a nightmare over USB for a NAS, in my experience.

I was hoping this might be the year that I can finally get rid of the spinning rust. But looks like AI data centres had other ideas! :-)

However, I will say that if you just want to run some virtualized Linux servers or similar, then ex-lease micro PCs are a fantastic deal and can be fun to setup and learn Proxmox and Truenas etc..