Why do we have boogers?

Answer 1

All of our mucus membranes produce a (typically slimy) protective substance called, oddly enough, mucus. One of the several purposes of this substance is to 'catch' airborne contaminants. The old mucus from lung and sinus membranes is disposed and with it the contaminants are expelled - most (I think) gets swallowed, some spit out or blown out the nose. Some thickens (with trapped contaminants and/or gets dehydrated) and becomes uncomfortable hard or sticky deposits in the nose. Mucus in the lungs or throat does not generally get dehydrated because the environment stays moister than the outer nasal passages.