[Unclenick]

It probably speeds up the rate at which solvent migrates out of the powder (what you smell when open a powder container). The less water is dissolved in the air, the more capacity it has to take on something else. It will dry moisture content out of the powder, increasing its energy content per unit weight, though it doesn't affect volumetric measuring, AFAIK. This is one reason benchrest shooters rely on volumetric powder dispensing rather than charge weighing. Both permeation processes will take a long time if the powder is in a sealed plastic bottle, as the permeation rate for the plastic will be slow.

I don't entirely understand the effects of moisture on smokeless powder. You are warned that too much humidity is bad, yet Alliant keeps a sample of the first lot of Unique under distilled water to preserve it, and they dry out samples periodically to check for deterioration by testing. In about a century none has been detected. So, clearly, the water by itself is not the issue.