HDRI's are a necessity for Rendering. Why? Well, if you're a 3D artist and want to create a realistic, “photo-real” environment in your scene, you have to have HDR photos taken of the environment or location. This is necessary because when you use HDR photos in the scene, they provide the details that would be lost in a traditional environment map. A good Quality HDRI makes it easy to simulate Realistic environments Lighting in a scene (i.e. mountains, deserts, forests, etc).