It's not necessary but, sometimes, depending on how much wood is being drilled, what kind of screws are being used, or how powerful the drill is, you could have trouble screwing.
The screws could break, thus leaving you w/the need to either remove, break, or bang in the broken screw. Pre-drilling can help you locate joists when you're not able to readily determine where the joists are. (Especially when there are multiple layers of flooring down and the installation isn't on the joists.) By NOT securing all the layers to the joists, you risk differing 'degrees' of movement of the subflooring and and any additional layers that are installed.