חזקיה אמר למה היה משתנה להן מפני הזבין והמצורעין כדי שיהו יודעים אימתי יום ואימתי לילה.
Hezkiya said the reason Hashem put a cloud on the Mishkan by day, and a pillar of fire by night (see Exodus 40:38), was so that someone who contracted tzaraas, or a zav/zava (who became impure due to a bodily discharge), [can count their requisite seven days before they can complete their purification] so they would know when is the day and when is the night.
Why did they need this indicator? Couldn't they just use day and night/the sun and stars? The Zayis Raanan (ad. loc.) says something about it being so they would know exactly when it changed to night, but I'm not sure why that would be necessary knowledge.