The original motivation behind the Add content restriction technique is to reduce the number of available page layouts in the Mercury template.
The figure below shows the Add content dialog of OpenCms' page editor. In this special situation, the editor did navigate to the Template elements view in the dialog, and the editor is just about to add one new Mercury layout row to a page.
As one can see, Mercury offers a large variety of responsive layout rows to the user. There are sites where this variety makes sense. For other sites a limited number is better—for example, if the site is intended for mobile devices mainly.