You can put a marker with the needle size used on your crochet or knitting project if you interrupt it and let it rest. That way, you will not forget what size needle you used in your work when you later want to start knitting again.
The safety pin is in brass metal and fits needles up to 8 mm. This is only relevant if you want the marker to be on the needle. You can of course put it on your cable / stitch holder, or you can mount it on the knitting itself, which you naturally want to do when the work is at rest.