How & Why
How . . . A photographer does this by adding in subjects in the foreground, middle ground, and background. However you can have more than just three layers making it more immersive.
Why . . . Photographers do this so that the picture looks complete with the multiple layers. They also do this because the image will look more three-dimensional, which brings in a more real world aspect and way to study the photo.
E x a m p l e s o f L a y e r e d P i c t u r e s . . .

For this photo there are about four different layers. There is the house number/sign. the grass, the house, and the blurred trees in the background.
For this photo there are about five-ish layers. There is the wooden beam on the right side of the picture, the green vine hanging down, the bleachers, the children, and the wall as the ending point.


This image has about five layers. These include the blurred books at the bottom of the photo, another layer of clearer books, a ladder, shelves, and a window. These layers create what looks to be a three-dimensional room (deep).
This image has layers that include books, wood, multiple candles, a vase and plant, a fire place, and even a reflection. In total there are about seven layers that create depth.


This image has multiple layers that include a plant sticking out the side, the pavement, a water pump, the grass, a bench, a lamppost, a basketball net, trees, and of course the sky. In total there are about nine layers in this photo.
This photo has about three main layers but once you investigate it more you can see that the photo has about six. These layers include the blurred (fake) dragonfly, a pinkish purple hydrangea flower, a fire pit, the grass, the fence line, and finally the trees finishing off the layers.

F E A T U R E D I M A G E . . .
The image above is my favorite layered image and is also my featured image. I love this photo because of how specified some of the subjects are and how it follows some of the rule of thirds. This image pops with green and pink/purple and it also has a pleasing amount of depth and dimension.