I have a pattern for a handbag that instructs me to begin the bag at the top with circular needles and work 10 rounds in garter stitch...
If this is true when using circular needles:
You need to make a join between the 1st stitch that you cast on and the last stitch that you cast on. After that, you just keep knitting around in a spiral. You never have to purl because the "knit side" is always facing out towards you and the "purl side" is always on the inside of the tube.
Then WHY is there a need to knit a round and purl a round to achieve the garter stitch when using circular needles?