Do i honestly need a sponge filter?

everyone keeps saying i need one( patricularly one... yeah you! your always griping and groaning about it) but do i really need one? first off, i have this one plastic thing on the filter and the openings on it are about a centimeter wide and none of my fish are going to get caught into that. why would i need a sponge filter then? should i just keep the 3 bucks in my pocket?