Boiling water is boiling water is boiling water…to a point. Yes, water boils at 212°F. But we're cooking here, and that calls for nuance. Just ask anyone who has been forced to chew through pasta that's nowhere near al dente. Why does this tragedy sometimes happen? Because adding new ingredients to boiling water causes a slight drop in temperature, something that makes a difference when your pot of water is just barely boiling. A vigorous, rolling boil ensures ain't nothing going to affect that boil!
And here's a simmer. Notice: Active water with tiny bubbles rising to a staccato beat. That's how you'll want your soups, chilis, stews, and other slow-cooking meals to appear as they simmer away after an initial boil, building flavor.