What is the use of pin caution?

Pins have sharp points, so if you are cautious, you can avoid puncturing your skin. Possibly you intended to ask about pin cushions, not pin caution. A pin cushion is a little pillow into which pins can be stuck, in order to store them conveniently. The reason for using a pin cushion is that it positions the pins for you, with the head outward and the point inward, so they can be more easily grasped.