Humco holding group
Saltpeter is employed to preserve foods, to make powdered incense burn more evenly, and as an ingredient in Gunpowder. (which is Saltpeter, Charcoal, and Sulphur)
Saltpeter also has a reputation for diminishing the male sex drive when ingested, although this is not backed by scientific evidence.
Magically speaking, Saltpeter has a long, long, long history in hoodoo as an agent of magical protection in spritual baths and floor washes and as a shoe-sprinkle.
Also, because Saltpeter is an ingredient in Gunpowder, it is used in formulae which are hoped to produce quick ("explosive") action, such as Block-Buster.
In this capacity, it can be an ingredient in love spells or money spells where fast action is required. (There are also certain "explosive" spells that call for Gunpowder as an ingredient, and if you can't find gunpowder locally, you can use Saltpeter, Sulphur, and Charcoal to mix up your own Gunpowder for use in such spells.).