it is less expensive to build.
Square No, in general they are six rectangles. Only in special cases would they be squares.
A square.
A rectangular square
square
Nepal
A rectangular prism.... has four rectangular, and two square faces.
SpongeBob SquarePants is a square, but his pants is an rectangular shape.
For strength of brick constructions it is useful to have them partially overlapping. A rectangular shape makes it easier to have them partly overlapping.
A 3D shape could be:a square pyramida cubea rectangular block
A rectangular prism has six faces. Four of these must be rectangular: the other two may be square or rectangular.
Rectangular or square.
Square Pyramid