The more 'square' a building is, the greater the floor area it can contain for the walls around it. Also, the more shapes that there are in the building, the greater the length of walls it will need to enclose the given area.

Likewise, the more angles and corners there are, the more difficult it is to build and (potentially) the more it its likely to waste materials. Fortunately, this tends to be less with timber frame buildings than other forms of construction.