Tear film has 2 very important functions:
Visual:  ensure you have crisp and stable vision by keeping the surface of your eye moist, smooth and clear so that light can be easily focused onto the inside of your eye.  If you ever felt letters are dancing around, you have experienced unstable tear film.
Protective:  maintain the integrity of the cornea so that you are not bothered by wind, dust, air conditioning and vents.  Tear film help wash away foreign matter, rinse away dust particles and protect your eyes from bacteria that can cause infections.
Without a stable tear film, you will experience dry eye symptoms. The eyes will have more difficultly functioning normally and keeping your vision clear in all types of environmental conditions, regardless of how high/low the humidity is or how hot/cold the temperature gets.