You are partly correct. They are not needed to get the abs to show, lowering your bf% as you said will reveal the 6 pack that everybody has. However, ab exercises are not meant to help your abs show they are meant to increase their function. Having a strong core is a must for any big compound lift. Remember there is no such thing as spot reduction, so working your abs with crunches and sit ups is not gonna burn the fat around your abs. Your body decides where to take the fat from.