The shapes of the crystals affects the taste. If the salt is going to be dissolved, such as in pasta water, then grain-shape doesn't matter. Use the cheapest salt. But if the salt is going onto the surface of a food, such as in popcorn or a steak, then fresh-ground salt gives a more salty flavor per gram used. The salt grains that come already ground have a smooth texture like the smooth pebbles in a creek. Fresh-grinding gives the crystals a jagged, irregular shape. This enhances the effect when they hit your taste buds.