Besides adding a height element to the yard design, ornamental grasses also add different textures, shapes, motion, and sounds. While this placement may not be the best example, it does give another angle on their use.
For some folks and designers, motion and sound are also considerations in their garden plans. While most folks don't really consider it, they are actually both effects associated to ponds and waterfalls that makes them a pleasure. Ornamental grasses, because of their height and texture, sway in the breeze for motion and create beautiful sound as the wind blows through them. So using ornamental grass types in your plans and ideas to create height will also add even more subtle and often overlooked elements that will most times enhance the design.