You will definitely find that newer models have better specs and wider compatibility with apps and games due to the newer version of Android, but ideas for leftover hardware:
Info screen/digital signs on a tight budget... If you have a community centre, charity shop or similar nearby, with an unused monitor or TV, you could have videos/Powerpoint presentations playing on a loop. Some setup required, but virtually no cost to the good cause
Simple work stations - If you know someone with kids at school and they don't have the funds for their own laptop/desktop PC, an old Android box could have a USB mouse and keyboard plugged in, and be used for homework (Google Docs or similar).
Depending on the app you use and webcam compatibility, as a video calling system. If elderly relatives don't want a smartphone, but would like to chat with family, you could have a go at setting up a simple interface for them. Definitely recommend a simple replacement launcher for this sort of thing. Just have shortcuts for the video app and perhaps the Play Store.
NAS - the performance might not be good enough for streaming multiple 4K streams across your house, but with a powered USB hub you could plug in a few external hard drives to an old Android box and share their files across your home network with SMB, uPnP, DLNA etc. The info and links found at
https://droidbox.co.uk/how-to/network-sharing-local-storage-for-kodi-and-dbmc/ and
https://droidbox.co.uk/how-to/sipping-power-quiet-characteristics-of-a-droidbox-as-a-server-vs-a-pc/ have a bit of info on this.
