In theory, putting rocks UNDER the steel ring itself allows more air to come into the fire pit area instead of the fire smothering for air. (In theory, air can draw from the bottom of the ring causing a better draft, thus creating a stronger fire). Actually, I think it's just inexperience on the camper's part and green wood.
What's sad then, rather than taking the rocks out of the fire pit when done and letting the ring rest back on the ground, they simply kick the rocks on into the fire pit leaving them .... yes ... making it more difficult on the next person to use the fire pit cleaning out the ash.
But, I'll take rocks over cigarette butts any day. Anyone who throws cigarette butts into a fire pit needs their own butt kicked! They don't burn, and they are NOT bio-degradable. Someone ELSE has to eventually pick them up. Nasty A-holes!