The actual biggest advantage is covering 1st base on any ground ball. for a lefty they simply side step, slap their left leg on the bag and stretch out their right arm for the catch. For a righty, in order to stretch (as all 1b are taught to do on groundballs to 3rd or short) you have to put your right leg on first base.
This causes a few problems for righties. Firstly, its and extra step to the base, secondly its not really a fluid motion and thrid you end up actually having to turn your body which greatly reduces your ability to field errant throws up the first baseline.
Before you post, stand up and try covering with your left foot on the bag and right arm out then repeat trying to go to your right foot. Then post up and say thanks.
The only position on the diamond (other than pitcher) that is truly interchangable is CF. Many coaches and players feel very strongly about corner OFs, some like the gloves to the gap and other like gloves to the line but most feel strongly about it either way. Thats another debate in itself but there is a reason right handed 1B are very rare.