Salt pills, more commonly known as salt tablets, are usually sold as a dietary supplement in the form of buffered sodium. Their purpose is to replace a suspected loss of sodium through sweat during rigorous physical activity over a long period of time. Similarly, those consuming a pure vegan diet may also choose to supplement their sodium intake. It is unlikely that the average person, whether vegan, athletic, or not, will receive advice from a medical professional to take supplemental salt, however.
Salt tablets are also used for water softener purpose, in looking for a water softener there are advantages to using tablet salt instead of salt blocks. For one, block salt is usually more expensive, and comes in large blocks, which may not fit into all water softeners. Tablets are easy to handle, leave no mess behind and can be used in any water softener. Tablets also easier to purchase, whereas block salt are supplied by two dealers globally.
Here you can find some samples.