They fit the most common applications without having to be cut. Eight foot is the standard size for height in most rooms in residential and business buildings in the US. Not having to cut them at the job site saves a lot of time! basically to accomodate 2- 4 foot pcs of wallboard plus 1/2 to 3/4 ceiling drywall or standard 8 foot paneling. 12 foot ceilings are also common in upgrade or older homes The goal is to make installation of 4'x8' drywall panels easier. A typical wall includes studs (92-5/8" if pre-cut), a sill plate (1-1/2" thick) and a double top plate (3" thick). Total framing height = 97-1/8" (8'-1 1/8"). The ceiling drywall gets installed first and is normally 1/2" thick, or 5/8" where fire-rating is required. That leaves 8'-0 1/2" or 8'-0 5/8" of wall height to install the drywall panels without cutting or leaving oversized gaps.