Knitting a flat piece on circular needles?

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.

In addition to the width problem, they also figure it would be heavy and cause lots of fatigue if you knitted it on straight needles. As for using circs, it’s really pretty much the same as knitting on straight needles. Each time you finish a row you’ll just bring the now empty left needle around and it becomes your right needle.

You use circulars because the piece is too long to squish onto 14" sticks. Or maybe it’s too heavy (which I don’t understand unless you happen to knit standing up).

Don’t join the ends. When you get to the end of the row turn just like when you’re knitting on straight needles.

Pretend you have two straights tied tog by a long string. Switch hands when you finish a row.