For seat cushions we usually wrap the foam core with a soft material such as polyester or a down/feather filled envelope. The purpose of this is to tighten the cover and eliminate wrinkles. However, some furniture styles require a leaner, flatter look, and because foam is cut with a straight blade and has perfectly flat surfaces, it can be a challenge to fill a cover with just foam and have the cover be snug in all directions.
The covers shown here have foam only inserts. To make the covers tight, we cut the foam about a half an inch thicker than the boxing panel on the cover. Not all covers will allow for this approach though.