Some food items left behind can actually cause damage to the local wildlife. Chicken bones will splinter and can cause distress or death in local wildlife. Food left behind attracts insects - stinging ones. When I was a park employee, we would often be forced to douse a fire pit full of garbage with gasoline and light it. This was the only way to clear the fire pits of wasps so we could remove the debre people had left behind.
Paper is a terrible litter problem found in most parks.
About one quarter of the paper I picked up while working in the parks had either a credit card number on it or a name. Be smart and leave your parks clean. Most people know the basic rule of camping. Leave your site cleaner than you found it. Camping fees have gotten out of hand in many places - but I assure you that the employees who will clean your campsite are not benefiting from these cost increases.