Cooking risotto seems to differ from regular rice in two important ways:
What is the effect of these two steps chemically on the rice that yields such a different product than just throwing it in boiling water and covering?
I always thought, and one of my molecular cooking books indicates that, smaller batches kept the heat higher (less temperature shock), and the scraping released starch. The article Key success factor for tasty and creamy risotto on ideas-smart.com indicates something slightly different:
Note that the instructions normally suggest that each batch of the stock needs to be absorbed by rice. In reality, something else is happening. Most of the water actually evaporates. The remaining “mash” gets warmer than boiling point. This speeds up the chemical processes, which create the intense taste of the risotto. These chemical processes are essentially the same as when browning meat.
Other reason for pouring the stock in small batches and stirring constantly is to let loose starch from the rice kernels. Starch makes the mixture creamy. So the tedious process of adding stock in small batches and stirring constantly is the key to having tasty and creamy risotto. Only the reason for doing this, which most recipes give, is often misleading.
This matches personal experience and some (other) of my cook books.
Amplifying that answer, I think the main thing is simply friction. If you dump the rice in all the stock at once, the grains don't rub up against each other and release starch. You want a thick mass that when stirred creates friction and loosens the surface starch of each grain to produce the creamy texture.
