This probably comes up a bit but what is the advantage of a CSS framework that provides a heavily pixel focused grid system for quick prototyping over say table

This probably comes up a bit but what is the advantage of a CSS framework that provides a heavily pixel focused grid system for quick prototyping over say tables? I prefer CSS by far to tables (so much that I don't use tables for layout anymore) but it seems like these grids are just CSS tables, and with the overhead etc. if you are designing in a table mentality you may as well use tables.