A pivot point is a specific position in 3D space that is used as a reference for the transformations of objects. All objects (curves, surfaces, groups) have pivot points.
When you group objects in Maya, a new node called a parent node is created for the group of objects. The pivot point for the group’s parent node is placed at the origin (0, 0, 0). This is useful if you later want to duplicate and rotate the objects radially (that is, in a circular fashion around the pivot).
Maya groups the objects together in a hierarchy and the pivot point is positioned at the origin. (When the pivot point is relocated to the origin the Move Tool manipulator for the selected group appears at the origin.)
With the two columns grouped, you then duplicate the columns with a rotation option, the groups will duplicate and rotate around the pivot point at the origin (0, 0, 0).
To duplicate and rotate the group
The columns are duplicated and rotated by 90 degrees with each copy.