As photographers, our goal is to get people to look at and be interested in our pictures. We can use color and light, focus and composition to draw people into our pictures. But one of the easiest ways to get the viewer interested in a picture is to have a person in that photo.
You can see evidence of this in everything from magazines and books, to the Internet and billboards. Having a person in the photo draws the viewer in, even if that person is unrecognizable. How many photos have you seen of a mountain with a climber silhouetted on the peak? How many shots have you seen of a trail in the woods with a biker riding down the path. Without that human figure it would be just another pretty picture. With a person in it, interest has been added. The viewer pictures himself/herself in that spot, and there's an emotional connection. You can do the same thing with pictures you shoot.
How many times have you happened upon a location with a great view? Next time, see if there's a way to add a person to that scene. Sometimes it's just a matter of waiting until someone walks by. Other times you can ask a person you're with to be that extra element, to give that added interest to the scene by stepping into it. There are even times when by using a tripod and the camera's self-timer you can be that subject.
An image often has more impact there is one figure present. By having one person, rather than a group, it's easier for the viewer to imagine themselves as that individual.
You'll also find that if the person's face is visible and recognizable, the viewer's experience will be different. No longer will they think of themselves there, but they'll begin to wonder who that person is.
And, if there's expression in the photo, the viewer will try to imagine what emotions or feelings that person is having. You'll quickly realize that photos that show emotion are more powerful than those that don't. That's because the viewer will in some way share in that emotion.
People photographs have always been among the most powerful images that photographers can create. Understanding why, and applying that to your own photography, is a great way to improve the impact of your own pictures.