How old are we talking?
- Anything before core iX series is not recommended to be used as a server (missing instruction sets, low efficiency etc.).It could still be used for fun projects like installing gentoo or old redhat with plasma 2.
- If you have Core iX cpu (preferably 3rd gen or newer) you xan host some services, but look into c/selfhosted if you're interested in that.
- You could also experiment with Kubernetes and combine lots of bad PCs into one less bad PC.
In the end PCs are useful only if you can run useful sodtware on them, but besides nostalgia there ain't much use I see in them.