As fuel is drawn from or added to the fuel tank, there needs to be a way for the air to escape to make room for the fuel, or to enter the tank to replace the fuel that has been used. If this vent is left closed while the outboard is running air won't be able to replace the used fuel causing the tank to form a partial vacuum inside and collapse in on itself. The vacuum in the tank makes it harder and harder for the fuel pump to draw fuel out until it can no longer overcome the vacuum and the engine stalls. If you see the top of your fuel tank sucked downwards after running or expanding like a balloon when it heats up in the sun your breather is either closed or blocked.