I dont know if it works the same for the Berry's, being coated or plated or whatever, but, generally a hollow base allows it to expand under pressure to create a better seal with the barrel. If it really makes a diff nowadays I couldnt say, but thats why the mini-ball, from the civil war era I believe, had a hollow base.
I suppose it could also give slight increase in capacity for same weight bullet, but thats just a guess.