If a plant has a deficiency in Magnesium or Sulfur, epsom salt (Magnesium sulfate) would act as a fertilizer, but those two elements and all other necessary nutrients would also be provided by a complete fertilizer. Perhaps in situations where a plant is not otherwise fertilized, epsom salt provides that benefit. However, many claims of 'magic bullet' remedies involving epsom salts either haven't been tested, or else have been tested and no benefit was seen.
Here is a good article that tries to separate fact from fiction regarding epsom salt.
https://s3.wp.wsu.edu/uploads/sites/403/2015/03/epsom-salts.pdf