This allows games to simulate how light behaves in the real world. While the original DXR allowed for ray tracing, Tier 1.1 makes it much more efficient and flexible. It allows for "inline ray tracing," giving developers more control over how rays are traced, which improves performance in complex scenes.
This is a performance optimization feature. Instead of shading every single pixel on the screen at the same level of detail, VRS allows the GPU to focus its power on the most important parts of the image.