Mucin seems to be a kind of throwback to our earliest evolution. When fish first started walking the earth, setting the stage for what would be a diverse population of land creatures, they needed to keep certain parts of their body moist. The creation of mucin makes it possible to keep us from drying out.
There is no coincidence that the only planet we know of that has liquid water on it has life on it. Water is necessary for life as we know it, not just water frozen as ice, but in liquid form. That is why life started in the oceans. Because of this mucin keeps us from being bound to the waters of the earth.