Batting serves a couple of different purposes. First, it minimizes the friction between the fabric and the foam so that the cushion is more comfortable and doesn't "stick" as you sit on it. Batting also adds "fluffiness" to your cushion by smoothing and rounding out the harsh edges of the foam and filling in any gaps and wrinkles in the corners of your cushion. That said, you don't NEED batting when making a bench cushion, but I strongly recommend using it!