When you don't know where you're going to wind up, don't know if it'll even be capable of supporting life let alone inhabited, but are offered the choice of a single Chaos God's boon, which would you choose? Slaanesh sings to the flesh, Khorne screams his might, and Tzeentch whispers his trickery, but only the Plaguefather offers a certainty of safety no matter what you face.