Most people don't purl at the exact same tension as they knit ... so if you knit your swatch flat (where you would have to purl the WS rows) and then proceed to knit your actual project in the round (where you would be knitting all rows) your gauge would likely be a little off which will affect the fit of your piece.
Also, I could be mistaken but I think Amy usually does most of the KH videos.
