Simple enough: You tuck this low between your pectorals, right on your xiphoid process (the tip of your sternum) and it keeps the head of your lav from rubbing on your skin or shirt, supports the cable, gives a good flat surface for sticky tape; all while hiding the microphone under nearly all types of clothing and wearer positions.
For recording where you need the mic totally hidden and hair-clipped or prop-mounted isn't an option (and the wearer has at least a shirt on) this is your best bet.