I have two sizes, small gold ones and large silver ones. They are different from safety pins, because there’s no loop at the bottom.
To Count While Casting On
I use them to mark groups of stitches as I cast on. (I only use circular needles, so even the small ones fit over the cables.)
To Keep Track of Shaping
I use the gold ones to show shaping. For example, as I increase a sleeve from cuff to underarm, I put a pin right in the increased stitch. If I’m increasing every five rows, then when there are four rows above the marked stitch, it’s time to increase again.
I leave them in until all the increases have been made. If I run low on gold pins, then I put a silver one in where the tenth gold one is and can remove the first nine gold ones.