Salt is very hydrophilic, I believe this means that it's very difficult to get the last bit of water out. Further, if you leave dry salt out in a humid environment and it naturally pulls humidity out of the air.
Much of the price of that is coming from not breaking the formed patterns during harvest.
edit: Or here Aljazeera https://www.youtube.com/watch?v=PgDiW2ELx7k