The simple answer is that it doesn't. If the sole exhaust fan is a 1200 RPM 80mm the amount of air it can exhaust is the amount of air it can move. Period.
However, with positive pressure the air the exhaust fan(s) will move is the air it has easiest access to. Air in the corners, crevices, under the video card, &c will become pockets of stability where the air doesn't move because other air can be pulled into the exhaust easier. The air in these pockets gets heated by the nearby components and stays that way since it isn't being circulated and exhausted.
With negative pressure those pockets of stability disappear. The exhaust is working harder than the intake, basically creating a vacuum inside the case, so those pockets of stability can't form. And that in turn ensures that all the air in the case is being circulated and heat doesn't build up around any single component--heat source.
There are, of course, other ways to ensure that pockets of stability don't form where you really don't want them (see for example Lian Li's side intake just under the video card on their newer cases), but as a general principle and without going to additional effort, negative pressure will provide a cooler case interior.
The optimum configuration can be different in every situation. If the only real pocket of stability is in the top front of your case and there aren't any components there (heat rises so it will try to go up an since the PSU is generally toward the back the top front is a place heat collects) you might choose to ignore it. I would. If, however, the overpressure from a positive pressure system is creating a pocket of stagnant air around the hard drives and they're running warmer than they would with proper circulation I'd work on eliminating that pocket of stability and negative pressure is the first thing I'd try. It's all a question of tuning--there is no "right" answer that always applies.
If I haven't explained this well enough, i.e, you have questions, please ask for clarification.
Marco