Hello Quattro:
Thanks for inquiring and I will try to give you my best answer.
1. Check valves are not dependable because they are in the flow of the waste and can become stuck or 'frozen'.
2. If the check valve is frozen and the sink drain is backed up, just into the disposal, it can be siphoned back into the dishwasher; its called backsiphonage.
3. Should the sink be backed up into the bowl and the check valve is frozen the pressure of the water could push back into the dishwasher; its called backflow.
4. The best method to prevent backflow or backsiphonage is with an air gap, the second best method, speaking of water supply lines now, is a reduced pressure principle backflow preventer. The latter is a detailed, two vlalve aparatus with a middle chamber that can open and allow the water to run into an open receptacle of some sort (like a floor drain).
5. We think of plumbing as 'cut a few pieces of pipe and glue them togather' while we think of electricty as something that can shock you and even kill you. The fact is, more people have died from sewer and fresh water problems than by elecrical shock. Case in point, the Black Plague of London, Rome probably fell in some respects because they invented lead pipe for the fresh water supply. Lead poisioning may have affected many brains, even fatally.
6. You have many 'air gaps' in your home, each faucet has an outlet that can't be reached by the water in the tub or sink without running over the top. Your ice maker has one to prevent freezing the water supply. Then there are other connections that have vacum breakers (the third best method of backflow prevention) such as your commodes.
7. Many municipalities, such as Louisville, KY require backflow prevention in order to protect their water supply.
8. If I sound like a teacher here, its because I am. However, the presentation here is in good faith and not meant to scold anyone; I just wanted to give a complete answer to your "I don't understand".
Glenn