Why is rice parboiled? And how does parboiling change rice?
The following excerpt answers questions 1 and 2. Additionally it reduces the cooking time for the parboiled rice.


  Also known as converted rice, parboiled rice has been pressure-steamed
  and then dried in its natural outer husk (which is later removed).
  This process hardens the starch in the grains so they remain firmer,
  less sticky, and separate when cooked. It also forces the vitamins and
  minerals from the outer layer of the grains into the endosperm, which
  is the part we eat. This makes parboiled rice a more nutritious option
  than regular (unenriched) white rice, which doesn't retain any of the
  goodness of the nutrient-rich husk.


There are several techniques you can use to prevent unprocessed cooked rice from sticking to each other:


Use medium to long grain rice.
Rinse the rice until the rinsing water is completely clear.
Use the Boil and Steam method to cook the rice. In this method boil the rice for 5-10 minutes. Then pour off the excess water and steam the rice for an additional 10 minutes. This method of cooking the rice reduces the most starch on the outside of the rice which is what causes the sticking.


Source:
The Science of Cooking Rice