having to do a firmware update on my soldering iron

You don’t. It works perfectly fine OOTB. Can’t speak for the Pinecil v2 with Bluetooth and the companion app but I have v1 and the software been stable and bug-free enough I’ve never even given a thought to updating the firmware on it

deleted by creator

Even if you need something just once, just install it and then uninstall it, takes like 10 seconds.

apt install foo && apt remove foo

That’s essentially what nix-shell -p does. Not a special feature of nix, just nix’s way of doing the above.

Actually using it though is pretty convenient; it disappears on its own when I exit the shell. I used it just the other day with nix-shell -p ventoy to install ventoy onto an ssd, I may not need that program again for years. Just used it with audible-cli to download my library and strip the DRM with ffmpeg. Probably won’t be needing that for a while either.

The other thing to keep in mind is that since Nix is meant to be declarative, everything goes in a config file, which screams semi-permenant. Having to do that with ventoy and audible-cli would just be pretty inconvenient. That’s why it exists; due to how Nix is, you need a subcommand for temporary one-off operations.