Horizontal incoming light/glare is reflected downward.
Work Step by Step
If bright light is coming into a window from around eye level, a normal, vertical store window reflects it back into a passerby's eyes, causing glare. (This is especially likely early or late in the day, when the sun is low in the sky.) However, if the store has windows that slant inward at the bottom, the light is instead reflected downward to the sidewalk.