Why do people suggest boiling rice before lowering the heat to low, when making steamed long grain rice? What happens if you don't wait for it to boil and go straight to simmer phase?
In cooking rice or many other grains (and even some grain based products such as pasta), two things actually happen:
- The starches absorb water; they are hydrated.
The boiling phase does both at the same time, very efficiently, but sufficient hydration happens before the rice is fully cooked through. So steaming to finish allows the rice to be fully cooked.
If you just simmer, things would simply take longer. The reason to turn the heat down is to not burn the rice at the bottom during the steaming phase.
You need to bring it to a boil first, to get it up to cooking temperature. If you start off with your hob on low (suitable for simmering), you'll waste a lot of time as it heats up.
Mirroring the other answers, what you're doing by bringing it to a boil under high heat is quickly beginning the cooking process. You need boiling water to cook, and that's all there is to it, so the faster you get the water there, the sooner you eat.
However, continuing to run your rice at a high rolling boil wastes water; too much of it escapes as steam, so the rice can't absorb it and break down the starches into a nice sticky layer for your stir-fry. So, you turn it down to a simmer (and cover it, which gives the steam a chance to re-condense on the lid). This is in contrast to pastas or vegetables, where the water's just a nice hot bath and there's no expectation of the food ever absorbing all the water it's cooking in.
If you remember your grade school chemistry, you'll remember that a state change (solid to liquid, liquid to gas) requires more energy than simply varying the temperature of a substance in the same state; it's why ice doesn't immediately completely melt in your drink, and it's why water doesn't immediately flash to steam when it hits boiling. The water's still at 211* and some of it is boiling (as evidenced by the bubbles), but less of it is getting enough energy from the heat to turn to steam, so more of it sticks around to absorb into the rice, gelatinizing the starches to soften it all the way through.