Thread:Sentry 616/@comment-25396609-20181109023454/@comment-25396609-20181126013938

From the SR 5 core rulebook:

Essence is your metahumanity encapsulated in a number. In Shadowrun, you have ample opportunities to alter your body or push it beyond its normal limits. Such actions often have a cost, and they can result in a loss of a portion of your metahumanity, which means a loss of Essence points. Each character starts with an Essence rating of 6, and it acts as a cap on the amount of alterations you can adopt. When it’s gone, it doesn’t come back. It also affects the Magic and Resonance attributes, as losses in Essence are reflected by losses in Magic and Resonance. While denizens of the Sixth World are accustomed to seeing a variety of augmentations and alterations to the metahuman form, the “uncanny valley” still exists. The uncanny valley is the disconcerting effect that happens when people see something that is almost, but not quite, metahuman. An animated cartoon with exaggerated features looks fine to metahuman eyes, but a computer program that closely, but not exactly, replicates human appearance is a troubling and unpleasant sight to most viewers. This is what happens when people see others with augmentations—on some level, people notice there is something less (or more) human about that, and they respond to it negatively. The change may not be exactly visible, but it is in some way noticeable—in one way or another, a person has become less human, and on some level other people notice this. This is why a character’s Essence is included in the calculation of their Social limit.

Basically, essence is what makes a person, well... a person. Chopping yourself up to strap robot pieces on eats away at that.