CAUSE IT LOOKS BAD***!!!!!
lol but seriously, it reduces the weight of the rod (not by much though).
like Tin said, its mainly personal preference, there really is no benefit, but i think most people like it because it looks attractive (at least thats why i like a split grip).
however, rods without a foregrip, not only do they reduce a minute amount of weight, but they allow your fingers of your left hand to be in contact with the blank when u palm the reel, and that gives you more sensativity.