Some themes come packaged with pre-defined sidebars that don’t leave you much room for expansion or growth. With Metrolium you can add an unlimited number of sidebars allowing you much greater flexibility and power with your widgets.
On top of that, you can add any of your sidebars to your pages, picking whether to place it on the left or right of the page. This ability to easily create new sidebars and switch them around in an instant gives you great flexibility.