Is the flapper valve important in the gas filler neck on a 2004 Toyota Prius?

generally, any valve or device located part way down the filler neck is to prevent stealing gasoline or prevent large objects from entering the neck. I'm assuming your talking about the little tin flapper that the gas nozzle pushes down as you insert it to fill your tank. any important part of the gas system would be in the tank itself or in the gas cap. I'm not a prius expert just sharing my general knowledge. Hope that helps.