That is just marketing and convenience, but there is no particular advantage in taking the combined products over the separate ones. In fact, it is more convenient to have everything in one tub, and there is no need to mix additional drinks. But there is not much of a difference. In the combined products they will usually calculate how much you need, and the dosages will be calculated in such way, that with a serving of whey protein you will get yourself a serving of BCAA.