Placing the main column on top of the sidebars has numerous advantages.

By using this layout, you can place the main column on top of the sidebars, instead of first having left and right column in your sourcecode.

The Contao CSS framework builds a page following the order left column | right column | main column. CSS will position those columns to follow the sequence left column | main column | right column.

Usually, your most important content is situated in the main column. So placing this uppermost would make sense, and enhance the accessibility of your page (SEO benefits, too).

By using the Holy Grail Layout from “A List Apart” (http://www.alistapart.com/articles/holygrail), you can do that (it's tricky, though).

This extensions provides a new frontend page template (“fe_*.tpl”), “fe_holygrail.tpl”, which you can select in your page layout. Whenever this, or another one with the prefix “fe_holygrail” is chosen, the extension takes your column settings, and generates a new CSS block for the header of you page. And, of course, there is a CSS file replacing “system/contao.css”, doing all the basic work.