The air that organisms breathe contains particulate matter such as dust, dirt, viral particles, and bacteria that can damage the lungs or trigger allergic immune responses.
Mucus is one of your body's mechanisms for collecting all the bad stuff (like dust and germs) and keeping it from traveling in your body.
Think of the snot in your nose, your nose runs when there's a lot of dust in the air, making you sneeze. What else makes you sneeze? When your body senses that there are germs in your nose it needs to expel (get rid of). This is why your snot changes color when you're more sick (like when you have a cold); there will be more germs in the mucus.
