What is the purpose of cooking byriani in stages?
A risotto and a pilaf are very different dishes from what is happening during the cooking process. In a risotto, you want the starch to come out of the rice to make the "sauce", whereas in a pilaf the rice is generally soaked, washed, then pre-fried in oil or butter to prevent excess starch causing the grains to stick together, or indeed a sauce forming. While both can be cooked in one pot, the risotto will be more liquid and starchy than the pilaf, which should have clearly separate, dryish, but buttery, grains. 

The same applies to a Byriani. If you were to add uncooked rice to the pan, the meat, veg, juices etc. and then seal and cook, the result would be more like a risotto and the rice stodgy. Byriani generally uses partially or fully cooked rice for this reason, resulting in more separate grains.