What is it used for?
Table salt is essential for human life. A large amount of the comercially mined rock salt is prepared for human consumption. Rock salt is also applied to road beds in cold climates to help reduce the freezing point of water on the road, thereby allowing it to not freeze-over at 0 deg. C (although i have no first-hand knowledge of this since i live in hawaii).
Why is it important?
As i mentioned above, table salt is essential for human life.
In addition, the formation of rock salt deposits from the world's oceans is the major mechanism through which the ocean's can regulate their nacl. Content, thereby not becoming "overly" salty.
Salt formations are also important in the rock record because they form one of the most important "traps" for petroleum in the crust. Geologists often use a variety of techniques to try and locate salt domes in the subsurface when they are exploring for new oil and gas resources.
