Why is it important to drain the air tanks on an air brake system, and what can happen if you don't?

AAMVA CDL Manual, Section 5, subsection 5.1.4 (p. 5-1), 'Air Tank Drains'; 49 CFR 383.111(b)(2), 'the dangers of contaminated air supply (dirt, moisture, and oil)'

Frozen or contaminated valves and lines can cause a partial or total loss of braking in exactly the conditions, cold weather, where reliable brakes matter most. This is a federally required knowledge-test topic because contaminated air is a real and preventable cause of air brake failure.