Its purpose

The Behelit is actually a key used by mankind to summon the God Hand, a group of immensely powerful, god-like demons, themselves the servants of the dark God that governs the universe. A Behelit is not truly found or kept by someone, rather it finds its way to the one fated to possess it through the manipulation of causality, and tempts the person with the opportunity to be free of their pitiful situation and have their inner most desire realized.

The Behelits meant to transform the one who possessed it at the right time into Apostles. Apostles, which are humans who have offered their most precious possessions in exchange to escape their horrendous fate, can transform into hideous creatures (a bizarre manifestation of their ego) and bow down to the God Hand.