They're especially prized for protection from law enforcement - many of you have shared your experiences with their efficacy for this.
I carry a few on me at all times - in my pockets, hat, shoes, car, etc.
Traditionally, a juke joint/illegal bar, bootlegger, cat house/brothel, smuggler, or anyone else working in clandestine or illicit fields would set 9 of these in their doorframe with nails bent in an X pattern.
When I fix these, I do so on the point of (through the spirit of) Coyote. Many indigenous tribes gave their war scouts coyote medicine because the war scouts' role was to move and observe unseen, to pass through difficult, blocked, or watched terrain, and to protect the tribe from hidden dangers.