Why boil rice first when steaming rice?
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.
Cooking.


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.