Salt is hygroscopic, meaning it naturally attracts and holds moisture from the air. That is why salt can become clumpy in humid conditions, and the same property can be useful inside a car.
By absorbing some of the moisture in the cabin, salt may help reduce condensation on the windows. This can be useful when temperature changes cause the glass to fog up.
The effect is gradual and simple, not a complete replacement for proper ventilation or fixing leaks. Still, keeping a small container of salt in the car may help the interior feel drier and the windows stay clearer.
