True story-- my friend started knitting a sweater a several years ago and put it down for whatever reason. Recently she decided to finally finish it. All it needed was one sleeve and a button band I believe. So she starts knitting the sleeve and it looks wrong and the gauge is way off even though she's using the same yarn and needle size. So after asking us (knitting group) what we thought she went down a needle size. It wasn't enough different so she went down another size. Her gauge had changed so much over time and experience she ended up using 2 needle sizes smaller to make gauge.
My experience-- I made a swatch and cast on a sweater. 6" up from the bottom I realized it was going to be way too large. Had to frog and reconfigure and started over. It fits pretty well now.
What do these stories mean? The first story is how your gauge can change over time as you become more experienced. My story is how knitting a small gauge swatch isn't much knitting so once you get into the big garment and knit a lot more you relax and it can change. Your gauge can also tighten up if you knit when you're stressed out.
Gauge swatches are important because they can give you basic info..do you like the fabric? Do you like the way the yarn is knitting up? Does this yarn actually look better with a different needle size as in the ballband says DK, but it's knitting up more like worsted? AND if you knit the garment or object right away and don't set it aside and you knit a lot your gauge may work out fine and none of these things will happen to you.
Point is though...gauge swatches aren't always perfect for measuring garment size. And knitting a swatch thinking it will be useful for the next time you want to knit a garment/object with that yarn may or may not work out. They are useful for knowing if you liked the yarn and fabric, but you may need to knit another swatch to find what your gauge is now.