What exactly is it used for?

Self-application:

It allows you to blow the *rapé* yourself—directly, 

precisely, and with the necessary force.

One end is placed in the mouth and the other in the nostril. Control: It helps regulate the amount of powder entering each nostril. Ritual and mental purpose: It facilitates the clearing of dense energies, clears the mind, calms thoughts, and improves concentration during meditation.