Epsom is used to add sulfate and magnesium ions to brewing water. It is often used at half the levels of Gypsum. 1 gram in 1 gallon changes the salt levels by 103 ppm sulfate, 26 ppm Magnesium and adds 108 ppm to the hardness.
Promash is a useful utility to calculate water salts.
Epsom salt is also ideal as a nutrient source for growing hop plants.