This code is not there is global.css... In fact, you have to add it to be able to hide some columns according to the current page....
e.g. The below code will hide both left and right columns on the index page,
body#index #center_column {width:951px}
body#index #right_column #left_column {display:none}
and this code will hide the right column on the category page,
body#category #center_column {width:951px}
body#category #right_column {display:none}
and this code will hide the left column on the product page,
body#product #center_column {width:951px}
body#product #left_column {display:none}
If you want to hide a module on some pages then you can do so by mentioning those page names in Exceptions list...
Modules > Positions > Edit > Exceptions