This may be a silly question, but what is the point of eggcrate in a mattress?  Does it make for better airflow?  Is it always used as the top layer?

It will make a foam feel softer and reduce pressure points, because there is less foam pushing back on you.  Might increase airflow as well, but I am a little skeptical that it does much of that once you lay on it and it compresses.  Often on the top layer, but does not have to be.