I have a ten year old server that was once a thin client (think the precursor of mini-PCs of today). Could it be more energy efficient? Perhaps. But it's always been a good little server, and today it's my SyncThing "file server" ensuring that the latest copy of a document is always available right away on all clients. PC hardware of all kind of expensive these days, so I'm not jumping to replace it while it serves a useful purpose and the hardware is good enough for those tasks.

Maybe it can one day evolve to be a personal laptop that you don't need to share with your dad, or perhaps, a server of some kind. But what useful means is highly variable. It depends on your needs and the use cases.