It is hard to say why boulders and sculptural stone are sometimes incorporated into American landscapes. A long time ago, boulders were only left in landscapes if they were to big and heavy to move out of the way or break apart. Early American landscapes were designed to express dominance over nature by replacing as much of it as possible with unnaturally organized landscapes.
Slowly through history, less refined and more relaxed landscapes became more tolerable, and then became popular as an expression of rebellion to earlier formality. Nowadays, most landscapes are inevitably informal, partly because so many believe that informality is more natural, and partly because few landscape designers will design anything else. Simplicity and symmetry are passe.
This informality allowed for the incorporation of various elements from various styles of landscaping, regardless of how incompatible some of such elements were with each other. Boulders and sculptural stone that had been traditional with many Asian styles of landscape design were added to American landscapes in rather nontraditional fashion. It has been a slow process of evolution.
Boulders are obviously nothing like viable and dynamic plant material, although they do contribute form, texture and color to a landscape. Designers might say that bigger and sculptural boulders add drama without even trying. In some situations, boulders are as functional as they are aesthetically appealing. They can obstruct unwanted traffic or hold back soil that is at a higher elevation.
If they need not conform to any of the various Asian landscape design traditions, there are not many rules for the use of boulders and sculptural stone. Exotic stone that might be incompatible to big open landscapes where exposed endemic stone is visible nearby, might be just fine in enclosed gardens where there is no reference for what is natural.
The standard rule of burying as much as two thirds of a boulder to make it seem as a natural outcropping is only valid if it is intended to look like a natural outcropping.