Arch Linux is a rolling-release system that expects users to update regularly.
This can be a problem if you find yourself in possession of an older Arch Linux
machine that hasn't been kept up to date. In particular, if you'd like to back
up the contents of the system and reinstall, you may want certain tools available
to do it (in my case it was pigz).
However, if they tools aren't already installed, you're out of luck: Arch's package repositories store only very recent versions of every package.