Epsom salts were used for some essential minerals before we had so many commercially available fertilizers. The practice has probably been handed down.
Epsom salts give plants
magnesium & sulphur.
magnesium is essential for chlorophyll production. Lack will result in poor chlorophyll production & stunted plant
sulphur is essential for photosynthesis. The use of sunlight to transform carbon dioxide into sugars. Also, it is an important ingredient in a plants
' produced chemical resistance to pests.
Proper sulphur levels will result in better bloom & seed
You can use epsom salts. Or commercially available fertilizers & ammendments which will give a more complete diet for your plants
and result in greater health if used on a proper schedule.