• Servers!
• Plugged into a dumb TV to make it smarter than a smart TV
• A sandbox to let you play with distros or to test server deployments or get malware or whatever
• A backup that you could keep somewhere safe for when all your other devices are dying, with 2FA recovery stuff, private GPG keys and revocation certificates, or anything else you might need
• A glorified typewriter; a distraction free setup separate from your main computer where you focus on writing (or do that for any other activity really)

If you really have no use for them, selling them is a good idea. I am still using a 13 year old thonkpad that I bought second hand. I am really glad that someone who did not use it decided to sell it. It got me to switch to Linux, and I took it on many adventures.