Exactly... As most explosive fumes are heavier then air, if you just pumped more air in, it may not displace the heavier fumes. Sucking air out of the lowest points of the engine compartment is much more efficient.,. At least thats the way my mind [whats left of it] sees it !You want to suck air out of the lowest point of the bilge. While the pump is on, fresh air should come in through your vents.
Good luck!
slight correction... It should pull fumes from below any ignition source IE starter ..... It should be at the lowest point that is NOT likely to have bilge water.... If the end of the hose goes under water then it will cease to function