Sodium chloride, or salt, is essential for human life and, until canning and refrigeration were invented, was the main method of preservation of food. Needless to say, it has long been considered valuable.
To be “worth one’s salt” is to be worth one’s pay. Our word salary derives from the Latin salarium, (sal is the Latin word for salt). Some debates exist over the origin of the word salarium, but many scholars accept that it was the money allowed to Roman soldiers for the purchase of salt. Roman soldiers were not actually paid in salt, as others suggest. They were obliged to buy their own food, weapons, among others and had the cost of these deducted from their wages in advance.
Salt continues to be essential enough to feature in the language for centuries. Other phrases that existed even in the medieval times were: take with a grain of salt, the salt of the earth and below the salt.