They originally used them in race cars because at the time making rims wide enough in one piece was really difficult. This is like early 70's.
However as time wore on we can now make really wide rims in one piece, but some believe that they are not as strong as the same size rim which is 3 piece. This is because a split rim has the actual rim itself milled from billet as opposed to cast, therefore its stronger, hence a lot of racers still use them but they really aren't necessary on road cars, but BBS splitties look the dogs so they are of course necessary.
