This is an old trick we use to influence how Photoshop builds its color table when creating GIF images. We put a selection around the area of the image that’s most important to us (for example, if we had a product shot, we’d put a selection around it), then we’d convert to Index Color (by choosing it from the Mode submenu under the Image menu). Photoshop will look at the colors contained in your selection and build the Indexed Color Table giving preference to those colors. It’s another slick way to use less colors, creating a smaller file, but with a better-looking image.