While potassium nitrate may be boring in solution, the solid is a  critical oxidizing component of black powder gunpowder. In the past it was also used for burning fuse technologies including slow matches.

It readily precipitates and was widely "harvested" since the Late Middle Ages and Early Modern era through the 19th century from urine from which it was forced to crystallize in various odorous ways. Its common names include saltpeter, American English saltpeter, & Nitrate of potash.
KNO3 (aq)
KNO3 (s)