The main reasons you may want to use IB+ site rather than BA's is to redeem
1) Vueling flights
2) Non OW flights (eg Taca, but the redemption rates are awful.)
3) 'Turista' bucket seats (Y seats, but at a higher rate than regular Y inventory) and which may not be available on BA or other OW award inventories.
4) If you have a IB+ credit card and can take advantage of the 25%/50% off award redemption promotions.