Shin...
If you took a minute to check the FAQ and roadmap on their site. You would have your answer as there is a section that asks "why would I want linux on my GC?" and then the authors proceed to answer the question with a few potential uses.

Linux is a usefull OS to get running on any hardware. A gamecube is only $99 US now, thats pretty cheap for a box with hardware that packs a considerable punch. Some people could use it in a distributed computing network to enhance the collective processing power of the network.
This is mostly about using hardware you already have to do more.
In my case, my main PC is very very underpowered and ancient. It's a P2 400Mhz MMX.
My GC which costed me only $199 US when I bought it on release day (I think that was the release price?) is like twice as powerfull then my PC which costed me $2000 when I orriginally bought it.
I'm particularly fond of anime also, and I like watching some on my PC. But with some files, theres audio/video desync and so on, I have to rewind often just to resync it. It gets annoying after awile. Now, when GC linux is done, I'll be able to play the files that are on my PC's HD, over the network, and on the GameCube, they will be played on my TV screen, and I'll be able to use a wavebird controller as a remote.
Now before you say something extremely typical (as well as predictable) such as "buy a better pc!", may I remind you that not everyone has the money to spend on new stuff all the time. And what the hell would be the point, I already *HAVE* the hardware and the equipment needed to do this, I even have a keyboard that I got for PSO, so for me this would be a really inexpensive and very efficient solution. And I could save my money for other purchases.
Anyways, thats my story. And when GC-Linux is done, I'll live happily ever after.

Another obvious use is as a development environment, there are many people who are in the homebrew gamedev sceene, and developing on an actual console will give you a lot of nessesary experience if you ever decide to go into game programming as a carreer.
If you want more examples, then you can check out the site.

I'm done.