My answer to why I use a scale . . . I have an 8 cup graduated mixing bowl that I use. 13 gallons x .5 cup/gallon = 6.5 cups of salt. To save the trouble of leveling the entire batch, or each cup individually - I just know what 6.5 cups of salt weighs. In my case - 4lbs. Dump 4 lbs of salt in that mixing bowl and now I have no need to worry about the exactness of the volume measurement. I just sift that into the fresh circulating make up water and it comes in just about 1.024 sg. And I adjust from there depending on if I want to raise or lower the tanks SG.
The question of salt absorbing water should be a non-issue, at least I think so. But I do keep the Kent mix in the original bags, in a plastic container that has some of those desiccant packets, like you find in pill bottles, which seems to keep the salt dry and pourable.
So, I just find it easier to dump 4lbs of salt, rather than measuring out 6 1/2 cups.