If you are not totally grossed out yet, I'd recommend reading "Eating Snot – Socially Unacceptable but Common: Why?", a chapter in the book Consuming the Inedible: Neglected Dimensions of Food Choice. You can read most of that chapter on Google Books. Author Maria Jesus Portalatin focuses mainly on the better documented socio-cultural implications, but gets into a bit of the biology. One thing she points out, nasal mucus is about 95% water, so there's a possibility that you might expect more mucus eating in arid places. But nobody has ever done the studies necessary to test that hypothesis out. Her main hypothesis—also untested—is that eating mucus might help prime the body's immune system, allowing it to have more contact with weakened forms of potential pathogens so it can better detect and destroy those pathogens later. In other words, she thinks that eating your boogers is sort of like self-immunization.
I must read this book.