Chess demonstration boards (also called demo boards) are primarily used by chess teachers. The chess position being taught is displayed to the whole classroom. Most chess classes consist of no more than 20 students.
The standard demo board size of 27" or 28" is sufficient for most classes. Sometimes its helpful to use two boards at the same time. If you are teaching more than 20 students with only one demonstration board you'll want to consider the larger 36" board.