The Bit Depth of an image will always be determined by how many bits are used to define each pixel. If you have a greater bit depth, you are going to see more tones, both grayscale and in color. Psychologically, the human brain can associate lots of colors in an image as an image that is “high definition.” More shades means more noticeable detail that can be seen by the naked eye. Inversely, eyes can only decipher so much of a color spectrum at once, so lesser colors can also be easily read. For an image to have a smaller bit depth, it makes it easier to load on a website, keeping potential visitors around if a picture appears quickly.