A surprisingly handy tool for knitters!
I bought a model which was small, pocket-sized, extremely precise in small measurements, and with a fold-away display. I'm pretty sure the scale is made for drug dealers. I would be a little nervous if I got caught with it by the cops, because I bet it would take some explaining if they dug it out of my purse.
But I have to say, drug dealers face the same problems we knitters do: they need to know how much they have, and they need to know it pretty precisely!
You can also use a digital kitchen scale, or even a postal scale. And incidentally, the best way to test it is to use a new (i.e. not worn down) nickel. A nickel weighs exactly five grams. (One gram for every cent, if that helps you remember.)
The most common use for a yarn scale is to split a ball of sock yarn in half. Whether you knit from the toe up or the top down, it's nice to know exactly where the halfway mark is, so that you don't accidentally go over it when knitting the first sock of the pair. And of course, if you knit from the toe up, splitting the skein lets you use up every last yard without fear.
To split a ball of yarn, just set it on the scale and start winding it off. You may need to occasionally tap the scale if it goes to sleep.
But once you start looking for reasons to use it, you will find them everywhere. I recently did a yarn substitution that I was feeling uncertain about. I had a 100 gram skein of yarn, and I needed to knit a 36" piece with it. After I had knit 18" I weighed the remaining skein of yarn. I was halfway through the knitting, so I knew if I had at least half the skein left, I was in good shape. (I didn't - I only had 40 grams left - so I ordered more yarn.)
You can also use it to find out how many yards you have. I had a collection of partial skeins of the same yarn in the same color. I weighed them all, then did the math to find out how much of that yarn I REALLY had!