Used to determine ACTUAL Living NPC's. Spigot mistakenly inversed the conditions for LivingEntity, and
used LivingEntity for Insentient Entities, and named the actual EntityLiving class EntityInsentient.
This should of all been inversed on the implementation side. To make matters worse, Spigot never
exposed the differentiator that there are entities with AI that are not sentient/alive such as
Armor stands and Players are the only things that do not implement the REAL EntityLiving class (named Insentient internally)
This interface lets you identify NPC entities capable of sentience, and able to move about and react to the world.
Aikar