Because the world is gross and dirty. Your own skin is covered in a layer of dead skin cells, bacteria, pollutants, viruses and dirt. (In fact, every single day you shed about 50 million dead skin cells [source: Chudler].) And that layer will get thicker over time unless you take drastic action and wash your face twice a day. Washing our faces not only gets rid of various antigens (tiny unwanted foreign invaders), it also provides our skin much-needed hydration. (This is why it's important to moisturize your skin shortly after bathing; the moisturizer's function is to seal moisture within the skin.)