After looking into it I can’t see how they actually help. I used to think that Normal maps simply instructed the material to react to light, reflecting it toward an XYZ vector.
But apparently Normal maps add polygons to the model and move them in the world. It’s like the polygons are there all along. It uses tesselation. Which means that not only do those polygons ACTUALLY exist, they also have to be calculated for. I’ve seen this in wireframe. Additionally, a Normal Map has to be held in memory as it’s a texture.
So… why is a Normal Map better than just adding more polygons?