K2SO4 is used primarily for the potassium, not the sulphur. Plants do need sulphur, but not in macro quantities. It is a micro nutrient. Some people have a lot of nitrate in their tap water, or they have many big fish, who provide a lot of nitrate from their waste. Those people can use much less KNO3 than the EI tables recommend, and substitute K2SO4 for the missing K. That is why it is included in EI nutrient packages.