Technically, the Longevity Treasures are not a traditional weapons; they are rather, treasures with the shape of a weapon.
A Longevity Treasure's main use is to prolong one's life, it can strengthen the flesh and feed the Blood Energy of its possessor; it could even refine the Longevity Blood of a cultivator. It is also have a pivotal support role in combat because it could quickly replenish the Blood Energy that was depleted during battle. It could even provide a burst of strength for the cultivator, increasing his combat effectiveness. Every cultivator have at least one Longevity Treasure.