Yes, there's also the aesthetic reason for use of rock; I used to notice that some potted cactus (and succulents) in retail stores covered the soil surface with small rocks, pebbles, sand, gravel etc to give the plant a more dressed look. Nowadays I don't notice that practice as much . Sometimes I will pot up a lone succulent with a contrasting colored rock like bits of charcoal so that the succulent can become more of a focal point. You can use red lava rock or pea gravel or aquarium gravel to dress up individual specimens.
