Nic salt is the kind of nicotine found in the leaves of tobacco. The salts contain not only nicotine but numerous other organic components. Nic salts work more efficiently in delivering nicotine because of the addition of benzoic acid to nic salts. The acid reduces the pH, which means the hit gets smooth.
Nic salts stand out in the market now because of their better, faster nicotine delivery and satisfaction. Nic salts allow the nicotine to enter the bloodstream at a similar speed to traditional cigarettes.
Another reason would be the better shelf life. Nic salts are more chemically stable than freebase e-liquids, you can store them much longer and the nicotine spark will not fade away.