At some point, you will have a lot of weak essences in stock and find yourself in need of some greater essences. You don't have to trade your lesser essences to the nearest store : you can simply fuse them.
v0.5 update : You can now fuse any number of compatible essences.

The essences used in Enchantment can be freely transformed from and into essences of the same category.
This guide shows how to use the Altar to split one essence into two lesser essences of equivalent value.

The essences are one of the two principal components of the Enchantment skill. It's by harvesting and manipulating that the player can prepare future enchantments.
While other sources of essences will appear in future releases, at the moment disenchanting existing items is the principal way of gathering essences.