As objects gain distance from us, the amount of atmospheric particles between us and our subjects increases. As a result, our ability to perceive differences in tone and contrast is diminished at a distance. When using color, saturation will also decrease. The distant object appears to merge a little with the background. In landscape scenes this phenomenon is called aerial perspective. This effect is especially visible underwater because water is denser than air.
This is just one way to create depth in our images. There are quite a few more to be discussed later.