I have a pi which I use as an apple tv/firestick alternative which works very well and would be pretty pointless with a larger pc imo. Servers I dont do with small PIs but indeed old computers. I think all kinds of ultra movable devices will be good with PI and derivatives.
For folks that want to get into it: pine64 is open source but I havent tried it yet. Thinking of it though. They even have a watch.