Well, it's mainly a time saver. Unless there is something exclusive to normal mode that you want, doing hard mode first gives you double loot rolls. Now, wether or not those rolls turn into something worthwhile is up to RNGesus, but doing hard mode first just means you don't have to go through normal mode. Two for the price of one. No, your odds don't increase, it just gives you your two rolls for that character in one go instead of two trips through the raid.