Question

Why add salt to the water when cooking rice?

Answer 1

For rice it depends.  Cooking any starch in water will first cause the starch granules to swell and eventually tangle up with each other (the gelatinization).  Dissolving sugars or salts in the water slows down the process by raising the temperature the swelling starts.  While few prefer pasta as a stuck blob of strands, the same is not the case for rice.  I like my Basmati loose, but my risotto and sushi sticky, so salt may be required for Basmati and optional for Arborio.

Answer 2

Salt is not necessary when cooking rice, but can be added. Unless you add a ridiculous amount of salt it will not significantly change the boiling temperature or time.

Answer 3

There are two reasons for that.

1. To make rice tasty to eat
2. To make sure rice is cooked well and there's no uncooked rice left in the pan. 

Cooking any starch in water will first cause the starch granules to swell and eventually tangle up with each other (the gelatinization).