Gas vapor is much heavier than air, so it does not tend to find its way out of the bilge without forced ventillation. And opening the engine hatch is likely to not get the gasoline vapor out.

It takes only a couple of tablespoons in vapor form to detonate.

Starter and alternator are specially shielded to resist open sparking and other parts are beefed-up to Coast Guard Standards to prevent leakage or reduce sparking. Pliable marine fuel lines, for instance, are heavy-walled compared to automotive fuel lines.

When the boat is in motion a couple of air scoops force air thru tubes into the bilge at midships, and pass air thru the bilge, out the back vent.

Blower is to ventillate at low/no speed. Time is probably a CG standard engineering calculation for how long a particular fan must operate to move enough air volume to make the air non-explosive.