What is the purpose of draining moisture daily from the air brake reservoirs (purgado de estanques de aire) on heavy commercial vehicles?

1. To fill the windshield washer fluid tank
2. To remove accumulated water and oil condensation from compressed air, preventing rust, sludge buildup in valves, and frozen air lines in cold winter weather
3. To reduce vehicle gross weight by 500 kg
4. To clean the exhaust muffler