Why boil then simmer in many recipes, like soup?
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If you bring the liquid just to the point of boiling, when you reduce the heat it will continue to shimmer = simmer. If you don't get it hot enough initially, in my experience it may scald but not actually simmer.
susan g has a good point. And by bringing things to a boil then you are quickly bringing the temperature of the whole pot where you want it and then keeping it right about there by lowering the heat to a simmer, at which temperature it finishes cooking everything in the soup/stew/sauce/whatever.
Also, if the recipe uses flour to thicken it up then it needs to be as hot as a boil brings it too to actually thicken up and cook out the flour like flavor
I will add this, one should never boil a soup/stew if there is any type of protein in the dish. Boiling causing the liquid to be at least 212 degrees Fahrenheit, which is terrible for protein as it will only cause them to constrict, giving up their water (I.E. toughening them up). Boiling lentil or bean soups on the other hand is actually a good way to speed up cooking time.