The ability to reuse electronics for new purposes until they stop working to the point that they're irreparable or otherwise no longer fit for purpose. You can do a LOT with old computers and hardware, and you can often resurrect "broken" things relatively easily, if you can find parts. However, sometimes you do have to weigh this against the energy costs some old stuff has relative to new stuff if you don't have direct access to your own renewables.

My entire home network infrastructure is built out of old, repurposed, and/or second-hand equipment. My router is a broken netbook running OPNSense, my switch is an older Cisco small business switch. I picked up a used couple year old used consumer router and put OpenWRT on it and run it as a standalone access point. It supports Wifi 6, and I expect it to last at least a decade. The only thing I bought new was my modem a few years ago, because I refuse to rent from the ISP.

More directly relevant though is that I'm using an old low power (formerly) Windows 8 tablet from 2014 to run Home Assistant, which I'm using to optimize my energy usage bit by bit. It's a slow process because I don't want to run out and buy new stuff until existing stuff stops working, but I'm slowly expanding my network to observe my energy usage and put in place automation to reduce usage where it makes no sense to waste energy.

One thing I'd like to expand into is actually learning to solder and picking up some basic electrical engineering skills. So far this has mainly been hobbyist, replacing only parts with connectors. I'd like to be able to do even more.