A NAS is just an example, here's a better one; I love to use my old phones as wall mounted displays and controls for home assistant, or as remote music players plugged in to some speakers that I can hook into in music assistant. Some of my old phones are more than capable of this hardware wise but are locked to older versions of android and can't run anything built for a newer version, so they end up as ewaste intstead.
I think my next phone is going to be a fairphone or something for this reason.