Carpet stair tread provides traction which will prevent you from slipping from the stairs. It is a form of safety measure.
Carpet stair tread is designed to be used on specific surface and condition. In highly trafficked surface such as theatres , the carpet stair treads are sturdy so as to stand high heels , splashes of drinks and oil . There are multi-purpose carpet stair treads that can be used on any type of surface. Carpet stair tread are made different materials such as; rubber, metal, polypropylene, braided fabrics, plastics. They come in various colors. Some have different line contours to hide step details such as edges. There is also a special variant called glow-in-dark treads to lit dark stair area.
Traditional carpet stair treads are not necessarily runner carpets , they come as uniform single carpets placed on each stepping surface of the stairs. Most carpet stair tread do not cover the entire stair surface ,they usually cover the center of the stairs surface where the traffic is heavy.