Cast iron transmits heat much better than stainless steel and in a very uniform way. It preserves heat better. For all these reasons you need less heat to cook.
Even better. Cast iron leaks iron into foods and iron is good for the human body. If you need iron suplements because your are low in iron, cooking with cast iron you get much better test results. Instead, stainless steel although it is an alloy more stable than cast iron, it still leacks small amounts of nickle. Some people are allergic to nickle and in any case nickle is not good for the human body.