The age combined with their CPUs, which are low power and low computation force means they can often be replaced with better devices for little money, so in my opinion none.
Only use I could have fornit would be for occasional situations, like maybe flashing an ESP device over a serial interface.
More modern machine might lack the serial interface and at the same time I would have Ubuntu or Debian installed on the old machine, which IMHO is better for flashing.
I do not have to fight my Windows machine, which I use normally.
True, but if HA is running on a NUC, then it is probably running a hypervisor or it would be a waste of resources, and the hypervisor would be better for a music server.
Yeah, server. And this old stuff as player connected to that server
I use similar old shit to upgrade nice old HiFi amplifiers, which do not have any network functionality. And then give it away to friends and family
Some are very happy, they do not need to throw away older, but still perfect amps.
I also have a few Raspberry Pi 4 just laying around and did use them for Volumio but the fact that you had to pay a subscription for multi-room was a turn off so I moved on
to Linkplay devices for multiroom audio and never looked back.
Android TV boxes are so cheap now and the newer ones are alot faster
I am probably going to throw them away and keep the psu that is 19V 3.4A and can
power an NUC