brake fluid can get water in it. this will cause all sorts of problems.
it’s why some manufacturers say it should be changed. Others, like GM say it’s a life time fluid.
Without corrosion inhibitors, even oil will corrode. Oil has a natural water saturation level just like the air does. When the temperature falls oil, like air, can’t hold as much water as it did when it was warmer and the water present condenses out of the oil. Even if the oil is dried, when exposed to air again it will “suck” up water from the atmosphere until it reaches the saturation point at the current temperature.
Corrosion inhibitors are essential.