When you are working a project in the round it is very useful to swatch in the round. This is because frequently knitters gauges change between knit and purl and a swatch in the round will give you a more accurate gauge.
If however you did a true circular swatch it would need to be very large to join in the round. In this video I'll show you a little 'cheat' method of working a circular swatch flat. You can do it using a circular needle or double pointed needles, just slipping to the right end of the work and looping your yarn around the back to create a flat 'circular' swatch.