Landscape rocks will add interest to your garden. You will be able to plant pretty rockery style plants in and around same. A sloped garden with grass can be difficult to mow. By adding Rocks you will have a natural looking garden which should be easier to maintain.
Well you just add the rocks to the edge of your yard where you would pot your plants . So leave some space to add soil and add plants and then put rocks in front as a fence.
A line of rocks, backfilled with soil will provide a slightly level area for planting or grass, but gives a slight terraced look. Look at landscape design books for ideas.