Brightness will determine how an image is processed in regard to the color value of the pixels in the image.
Brightness values can range from 1 to 10.
Lower Brightness values will lighten the image prior to processing, so that less pixels will be included in the image processing.
If the quality of your images are dark, then you may want to brighten the image (lower Brightness value) prior to processing to filter out extraneous pixels.
If the quality of your images are light, then you may want to darken the image (higher Brightness value) prior to processing to include more pixels in the image.
OCRFolder converts all images to black and white prior to processing. So, Brightness is significant for color or grayscale images because brightness will determine if a color pixel is converted to black or to white.