Excerpts from an interesting article at The Olive Tree:
The dome-like shape which the cushions tend to take (made possible by an adaptation that makes all the plants in the clump grow upward at the same rate, so no one plant is high above all the others), and the closeness with which those plants grow, makes these clumps perfect heat traps. The temperature on or inside a cushion can be up to 15 °C more than the air temperature above it. The cushions are able to retain heat radiating up from the soil, as well as absorbing heat from the sun (a very dense, large, clump of green can get surprisingly warm on a sunny day at high altitude)...Via Boing Boing. The plant in the photo may or may not be technically a "cushion" plant. It was the only small dome I could find in my photos this morning (taken at a "barrens" area along the Wisconsin River). I don't know what it is.
The desirable microclimate created by the cushion is not just desirable for the plant itself – in fact these plants have been shown to provide habitat for other plants, which shelter under the canopy of the cushion, as well as a variety of microorganisms and arthropods. Studies on various species of cushion plant around the world have shown that, depending on the type of plant and the location, cushion plants may improve species diversity, richness, or evenness (often all three!) when compared to a similar site with no cushion.