What is an 'HDRI environment'?
Lighting is hard
When lighting a 3D scene for rendering (be it complex path tracing or much simpler shading in a realtime game) it needs a light definition for any shadows or interesting reflections to appear.
A scene can be lit using simple light sources like point- or sun lamps but that's not how light in real scenes behaves. Even though the sun in the sky might be the only obvious light source in an outdoor scene it is usually not enough to just add a single virtual sun because in reality the light bounces off of all parts of the environment until it hits the subject. Replicating this with actual scene geometry is extremely laborious and computationally intensive, especially if the final render is only supposed to contain a few reflections on - for example - a metallic object without any focus on the background.