The universe was constructed by the Creators for reasons known only to them and possibly revealed to the All-Mother when she was given her mission.
The Creators built the Primal Realms to filter the raw power of the Brilliant and the Dark so that they could touch each other without destroying the universe. Instead, in their filtered state when they are combined they create Primal Essence.
Primal Essence and the Planar Essences react here through unknown means to create Life Essence. When a creature is born it has a small spark of Life Essence in it. As it grows older, this spark grows more powerful. Finally, when it dies, the Life Essence is released. Unless it is captured through some means, it makes its way to the various deities where it becomes Divine Essence.
Eventually all Divine Essence flows to the All-Mother who uses it to accomplish the mission set to her by the Creators.