When baking bread and making croissant dough, salt serves a couple of important functions. It imparts flavour and it acts as a stabiliser, stopping the yeast proving too quickly. In cakes, the salt is weighed to balance the sugar and other additives.
When baking bread it is best to use sea salt, which will be organic, unrefined and have a better flavour. You will not need to crush sea salt flakes, as they will break down in the dough naturally. Rock salt will be too intense and the crystals will be too large and will have trouble breaking down within the dough. When making pastry, it is best to use good old-fashioned cooking salt as the crystals will dissolve easily into any mix and the potency will be consistent.