One of the biggest advantages of pipe over rectangular is when you need to flow through multiple angles you do not need to worry about valley angles which can impede flow if not designed properly.

The cross sectional area required must calculated for both to get the area required for proper flow. In other words a 12" pipe does not give you the same volume as a 12" square chute. ■

A further drawback of rectangular pipes is the tendency for fines to stick in square corners, particularly if the surface or yhe material is damp. This is because the surface area resisting slip is large in relation to the mass of material in the corner, although the effect can be mitigated by constructing a small radius instead of a sharp corner. ■