In many parts of the world, that's the hand you customarily wash your arse with. So, it's a practical measure, to stop you getting faecal matter on your food.
You got it backwards. You use your left hand for that and reserve the generally-more-dexterous right hand for everything else. That's why, for example, you shake hands with your right hand, and why in those parts of the world, doing pretty much anything with your left hand is still a grave offense.
