What is the biggest advantage to batting left?

I've noticed for the most part that a lot of the great players and home run hitters that a lot batted left. There is some balance though with both. 3 of the 6 players to hit 600 home runs batted left and three batted right (soon to be 4 when A-Rod becomes the 7th) Of course Babe Ruth,Barry Bonds and Ken Griffey Jr batted left and Hank Aaron, Willie Mays and Sammy Sosa batted right as does Alex Rodriguez. It might be that a lot of pitchers are right handed or that most ball parks benefit left hitters. Is there something else to it.