Why would I want a polygon structure if I'm just going to hide it?

The front end of the speed reduction crankshaft (1) and the tail end of the main shaft (3) are made by adopting the polygon structure and are same as the inner structures of the grooves (5) and (6).

The project with the ground in the shape of a twelve-sided polygon and of concrete structure is unfinished and has been abandoned for many years due to economic crisis.