If you're an American, born anytime after 1940, of middle-class indoctrination, you most likely lived a portion of your early life in some version of the tract house.
Vernacular suburban landscaping, as I remember it, mostly didn't do layers. You had lawns, trees, and something in between on the borders. The in between acting as a moat-like barrier between the house, street, and neighbors.
The photo above is from photographer Julia Baum. See her photo essay of maturing suburban homes here.
Layers give depth, illusions of space, and levels of interest that the suburban yards of my youth were crying out for. A challenge beyond the reach of a mere lawn and oleander border.
Layers give space a visual hierarchy, not just of height and planar discontinuity, but also guide the eye to new discoveries.
As the elevation rises, plantings help break through the linearity and offer many surprises..
The layering can be horizontal and vertical, linear and non-linear.
Brazilian landscape architect and multi-hyphenate artist, etc., Robert Burle Marx was the master of layering. Above, he also uses layers in a horizontal graphic way. Above and below, an experiment with the patterning with stones and gravel and juxtaposing different grasses.
Marx goes all flat in tile and stone on Rio de Janeiro's Avenida Atlantica.
Japanese architect Tadao Ando and the Modern Art Museum of Fort Worth: Only the squares of the structures, water, and concrete walkways are needed to complete the vision. Any other details would be superfluous.