As he even points out, this isn't the first time this has been done, but Quag7 has written up a very detailed, simple, and often hilarious (I feel the same way about Michael Bay!) guide for converting that old Apple ][e you've got sitting around into a Linux serial terminal. In this case, he runs a serial connection between a modern (everything is relative here) Gentoo distro and the ][e using ADTPro as a go-between. Hook up ADTPro on both, get the serial hooked up and running on both, install a term program on the ][e and then make sure everything can talk to each other (at 9600 baud-- remember baud? Oh those were the days), and voila, you've got an Apple ][e that can run a Linux distro. Why would you want to do this? No idea.
But why else do we do this kind of crazy stuff (like hook an Apple ][e up to a game machine, or even an actual Mac)? We do this, my friends, because we can.