lubrication, moisture retention, and trapping foreign bodies such as: dust, pollen, virus, bacteria, etc. protecting your lungs from a lot of undesirable things.

Read this article: https://kidshealth.org/en/kids/booger.html

It traps dust, microbes, and other stuff before the air makes it into the lungs.  The crud gets caught in the snot, and then expelled, helping to keep the lungs clean.

Pretty much the same as earwax.  It catches dirt and dust, then gets expelled naturally.  This helps keep our insides nice and healthy.

ELI5: what is the purpose of snot?