If you’re trying to show how big something is then scale is important. Often times I see landscape images where there is a rock or log in the foreground as an anchor and also to give some scale. But that can be misleading as you don’t know how big those items are to begin with. People are a great way to show scale. We know how big people usually are and thus we can visually understand when we somebody in a photograph how big or small the surrounding area is.
Lakes and mountains are great examples of wanting to show scale. You can easily take a D5, 24-70 AF-S get low and shoot up and make a lake look huge even if it’s not. But by shooting wit a long lens, compressing the background and using a person to show the scale you not only get a greater feel of depth but also size. My friend Mikey did this job perfectly at Lava Lake, not only because he is wearing a white hat, but he also shows just how big that mountain lake was.