To improve flavor (It should say that on the package). As you may know or already do, adding sugar or salt to certain strong flavors enhances the flavor a bit. For instance putting a little salt on a grape fruit or watermelon will make it taste sweeter. However that is only secondary to the real reason (It was a happy accident that we find that table salt tastes saltier with a little sugar added). The main reason is the chemistry of Table Salt. Table Salt contains sodium (pure salt) it also contains Potassium Iodide a form of salt, which was added to salt to give people this essential nutrient to prevent goiter (a thyroid issue). Potassium Iodine sublimes (evaporates) out of pure sodium, the addition of sugar (and sodium bicarbonate (baking soda), sodium carbonate, and sodium thiosulfate) all work to stabilize the Sodium/Potassium Iodine combination, meaning that the Potassium Iodine will not evaporate out of the salt. The amount of sugar added (usually in the form of corn syrup) is about (0.04%). Another ingredient used is Calcium Silicate this is to prevent clumping. Salt is hygroscopic - that means it absorbs water from the air. In humid areas if there was not calcium silicate in the salt the salt would cake or form clumps of hard crystals over time - much like the granulation of honey if it sits on the shelf. Unlike honey adding a wee bit of heat does not dissolve salt crystals, one would need to grind the salt again.