After an immortal crossed the barrier of Will they had access to another form of Essence, and if they wished they could choose to cultivate that Essence which was called Enlightened Essence.
There was not much difference between Pure Essence and Enlightened Essence, but a higher dimensional Immortal could have an additional stream of Essence to call upon, further deepening the power they could call upon.
The primary reason why Enlightened Essence was important to higher dimensional immortals was because it could close the gap in their powerset of the Immortal if they had naturally weak Pure Essence.
In the matters of Essence no one was born equal, the amount of Pure Essence available inside a dragon at birth was a million times greater than what was available to a human at birth. At the Enlightened Essence level, a human Immortal could finally have the chance to grow their Essence to levels that rivaled dragons.