What's so great about Knoppix?

I'm just wondering, because I keep hearing about it and I've read a little bit about it. From what I understand it's just an OS that runs off of a CD. A version of GNU/Linux, I guess.

So what makes it worthwhile? I mean, can you write data with it? Because it's on a CD...

Anyways, I'm a total newb when it comes to Linux and this Knoppix thing has been bugging me for a while.

WHAT MAKES IT SPECIAL!?

Isn't there a way to install Knoppix, so as to ditch the disc? Also here's a good quesion; on my desktop a Live CD won't work, but on my old Dimension computer it will run just fine. Why is this, and what can I do to change it? Is the reason I can't install Linux, and the reason the Live CD won't work somehow linked?