Knitting a flat piece on circular needles??

[Kaillech]

I have a simple pattern for a shawl that says to use circular needles, 29" circular 13s to be exact, and I'm not seeming to grasp how this works. It's a flat piece so why not just use straight needles? I'm guessing because the width is longer than straight needles can accomidate so now I'm rather confused on how to knit straight on circular needles. Can anyone shed some light on this? I'm sure it's probably really simply and once I get it it'll be a duh moment, but right now I can't figure it out to save my life.