Linux is an operating system. Once you have the operating system up and running you can get the device to do other things.
(for instance turn your xbox into a file server / web server / HTPC).
Once linux is running on the DS then you can make way for mp3 and videoplayback, smaller homebrew apps etc.
Since linux is so small and versatile, as an OS its an obvious choice to try and get up and running on these devices...
also, the Tux squad is a little obsessive.