You see this all the time in recipes for everything from soup to sauces: "Bring to a boil, then reduce to a simmer." Ever wonder why you should go to the trouble of bringing something to a boil if you only want to simmer it anyway? Here's why.
Reason #1: Speed
The biggest reason why recipes have you boil first, then reduce to a simmer is speed and efficiency. When you're bringing water, a soup, sauce, or any other liquid to a boil, you're usually doing so over high heat, which means you're getting maximum output from your burners. This quickly brings a liquid up to its boiling temperature, and from there, it's fairly easy (and quick) to scale back the heat and bring the liquid to a simmer.
If, on the other hand, you left the liquid over medium or low heat in an attempt to bring it to just a simmer and no more, it will take much longer to reach that same brisk simmer.
Reason #2: Control
Another reason has to do with control. When you bring food up to a boil, then scale it back to a simmer, you have much more direct control over that simmer. The liquid will respond more quickly and reliably to changes in temperature — you can adjust the heat to reduce the liquid to a slow simmer and then bump it up again to bring it to a rapid simmer, all fairly quickly.
Reason #3: Even Cooking
When a soup or sauce is boiling, you know that everything in the pot is at the same temperature, which means that it's all cooking at roughly the same rate. From there, you can reduce the soup to a simmer and know that everything is still cooking at basically the same rate. It removes the guesswork. You can also be confident that your food will stay simmering at approximately the same steady rate until you change the temperature again.