You can now see an RGB and a Luminosity histogram for any image on this site. Luminosity is a combination of R, G, and B values, and the formula used probably varies between cameras, Photoshop, and BirdPhotos.com. Click on the Image Sizes pulldown in the top right of any image page.
Why are histograms useful? Histograms are especially useful on a camera, where a photographer can quickly see if he/she is over or under-exposing the image. They can quickly show if an image has 'hot' pixels, or pixels whose R,G, and/or B values are out of bounds, or at the highest value of 255. Or 'cold' pixels (does anyone call them that?) whose value are the lowest value of 0. Histograms can also be used to see how well distributed the colors are.
Are histograms useful on BirdPhotos.com? Possibly in discussing the technical merits of an image. Or maybe just to show the world we have some serious programmers here.