Some years back I was having dinner with some friends in the pipe community and wondered aloud why British blends were so wet, to which a very well known blender replied, half facetiously, "because water is cheaper than tobacco".
Sometimes you will hear a blender refer to the importance of the water in how a blend ages.
That said, I think the truth is somewhat less mysterious or foreboding. Tobacco has to sit for an undetermined length of time before it is sold to a consumer. If it's in large bulk bags, that bag gets opened a number of times before the contents are used up. Besides, unless the plastic bag has been metallized, it's not impermeable and moisture is being lost during the whole time that it sits. So, my expectation is that the water is there to keep the contents from drying out before they are sold. Simple as that.