I just read a few articles about cooking rice targeted at Americans ("take one cup of rice...") and in many comments people mentioned adding butter when they cooked rice. Why? Basic rice is cooked with just water and salt. 
Well, because they like it, that's it.  In another form, it's like cooking the rice with tomato sauce - it absorbs better the taste compared to adding it at the end, there's no other benefit for the rice itself.  For example, sometimes I prepare whole grain rice with tomato sauce / vegetables broth and some chopped vegetables - it's much better than adding it all at the end of the process. 