Because the chaos god Slaanesh was born from the hedonistic excesses of the Eldar civilization, all Eldar souls are drawn to Slaanesh when their bodies perish. Since becoming the plaything of the universal personification of sado-masochism is undesirable, the Eldar needed a safe refuge. Each Eldar wears a soul stone at all times; should the Eldar die, his or her soul enters the soul stone instead of being loosed into the Warp, where it would be snapped up by Slaanesh.
In addition to providing a safe haven for Eldar souls, soul stones that already contain the essences of dead Eldar can be incorporated into Eldar technology in a variety of ways. Most soul stones on a Craftworld are linked to a network of soul stones called an Infinity Circuit that serves as a repository of Eldar knowledge. Soul stones can also be installed into artificial bodies, allowing deceased Eldar to continue to aid in the defense of the Craftworld.