Whether your are designing your own knitwear, or knitting someone else's pattern, a gauge swatch can be invaluable. This is not to say that you cannot design and knit without swatching first. For as long as there has been knitting, there have been people who picked up yarn and needles and simply jumped forth, feet first, into their projects.
That said, there are some distinctly useful reasons to consider knitting, washing and blocking a swatch of any yarn you plan to knit with, and if you love knitting (and I think you do) you can make this a fun aspect of the project.
When I think of designing I think of it like building with Legos. Each block can be a different size, so that 10 –1 inch wide blocks stacked next to each other will be an entirely different length than 10 - half inch blocks in the same arrangement. If you are planning to make your Barbie a fort, replete with moat and dragon, you'll need to make sure you build it large enough for her and her cavalcade of cannons (to keep the riff-raff out, of course.)
Once you know you have X number of stitches and Y number of rows per inch, you can easily determine how to decrease evenly from your hips to your waist, then back out for your chest, even if your measurements are wildly different than the average. Without those numbers, you might find yourself decreasing too quickly or not quickly enough and that either means a trip to the frog pond (rip-it, rip-it, rip-it) or an ill fitting garment.
Furthermore, knitting a gauge swatch gives you a chance to get to know your yarn; how it commingles with your needle choice, and gives you a chance to try the stitch pattern and see if it suits your taste. I can't tell you how many times I've realized that my needles were poorly matched to my yarn, occasionally with disastrous results (think rough wooden needles snagging smooth microfiber.)
Best to leave those discoveries to the swatch stage before you've begun knitting rows of 200 stitches.
Finally, as designers, you are not limited to the gauge and needles specified on the ball band. Your yarn may knit up with too much drape, or not enough, when knit at the specified gauge, but go up or down a few needle sizes and the fabric may be just what you hoped. Use your gauge swatch as a chance to find that perfect match between the two.
I could stand up on this soapbox all day, extolling the virtues and joys of knitting gauge swatches, as these are just the a few highlights, but I don't want to scare you all off yet.
My next post will show my theory in action! Stay tuned for my adventures in swatching.
PS. Go team CALMER!