That's the idea of having mucus in your nose. We use it to trap all the dust, pollen and junk that's floating in the air. Sometimes when you blow your nose, out comes gross black stuff. It's usually mucus with trapped dust. It's healthier to keep that kind of dirt outside of our bodies.
All mammals make mucus.
Cats do something special with theirs. It's probably a good idea to save this recipe. You Can never have too much thick, gooey, sticky, slimy stuff on hand, or at least the directions for making it.