A single repeated plant or color can have a lot of impact in landscape and garden design. While this example of mass repetition planting does a classy job of defining and bordering this garden path, it would also work well as a planting background, bed border, or even a display on its own.
In situations like this example, the color can help set the mood or atmosphere. Imagine the change if these flowers were a bright yellow Esperanza. The color and boundaries would feel closer. It may even have a more tropical feel.
For most folks, I think the phrase "mass planting" brings to mind a denser more colorful display. True, most mass plantings are generally a blanket of repeated single or multi colors. However, that doesn't make this example any less.
In this design, the repetition and mass planting of green with the purple being secondary, fits perfect to help create the character and atmosphere of the garden. Don't forget that green is a color too. And in creative landscape designs, texture is also an important element to consider.