Just about every knitting pattern out there mentions the gauge the knitter of the sample got when he or she knit the project and thus the gauge you ought to be getting to get the same results when you knit the project. And just about every knitting pattern out there advises knitters to check their gauge before casting on for the full project.
Knitting a gauge swatch can seem like a big waste of time (unless you belong to the camp that feels any time spent knitting is a good time), but even a small variation in your knitting versus what the designer intended can make a big difference when it comes to fit.
A Real-World Gauge Example
To illustrate this point, here are two garment pieces that I knit up using Stockinette Stitch worked in the round. Both pieces are worked with the same yarn and started with the same number of stitches: 100.
The top piece was worked on size 6 US (4 mm) knitting needles, while the bottom swatch was worked with size 7 US (4.5 mm) needles.
The gauge with size 6 needles is 20 stitches per 4 inches, while the gauge on size 7 needles is 17.5 per 4 inches. That translates to the difference between 5 and about 4.5 stitches per inch (4.3 to be precise). That doesn't sound like much when you think about it, but if you do the math, over 100 stitches that's the difference between a garment that's 20 inches around and one that's 22 or 23 inches around, which can make a big difference in how the garment fits and looks when you're wearing it.
And that's just a difference of half a stitch per inch. If you got up to a whole stitch difference, over 100 stitches you'd have a 5-inch difference in the measurement of your garment.
The moral of the story is, if you want your knits to fit, really do take the time to knit a swatch, block it or at least wash it how you intend to wash the finished garment, measure it properly and then determine if your gauge is appropriate for the pattern or if you need to adjust your needle size.