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.
This is a pretty broad sweeping generalization. Can you clarify your question?
Generally the fats that are used in regional cuisine are determined by availability, cost, and preference. The Pilaf Method for cooking rice (French Method) also incorporates butter into the cooking process but traditional risotto though not void of butter as based more broadly on the use of olive oil.