Here's the answer from someone who hates it, but understands that people love it:) :
You use 1 long circular needle. Because of the way you slide the sts around, it works for small, tubular knitting, such as socks or sleeves. With that method, you only need to own 1 needle of each size-- that long circular. Because that way you can knit flat, large tubes, and small tubes.
There are people love the technic, and love only having to own 1 needle per size. What drives me crazy about it, is just that-- the technic, and only owning 1 needle. Stopping and constantly sliding the needle and stitches slows me down. I learned to knit small tubes on dpns and go very fast that way. Also the cord of the needle gets in my way. And I love needles of all materials and shapes and sizes, so it would cut into my hobby of collecting;).
But have a look at the video and see what you think-- you might be one of the converts!