Facilitated diffusion is a means of passive transport; thus, it does not require the cell to expend any energy whatsoever to get a substance in/out of the cell. Basic diffusion through the membrane doesn't work for a lot of substances simply because they are too big to fit through the gaps between the phospholipids. Facilitated diffusion provides the cell with the ability to take in/expel molecules larger than the gaps between the phospholipids without expending any energy. Hope this helped, or maybe it's too descriptive?