What is the purpose of cooking byriani in stages? Risotto and pilaf are generally cooked as a one pot dish, where the aromatics are fried and then rice and cooking liquid are added.

Most byriani recipes are see the rice is part cooked separately and then layered on top of the sauce. Since its all mixed together at the end this seems to add complexity in two pots and having to drain the rice.

What's the advantage that this two step process adds?
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.