The biggest difference I find is that the designer who doesn't understand the guts of layout ends up with a site that is literally inflexible: it doesn't resize

The biggest difference I find is that the designer who doesn't understand the guts of layout ends up with a site that is literally inflexible: it doesn't resize well and is really hard to modify later. The general outdated-ness of many corporate sites can be tracked to the difficulty of getting a flexible layout in place that lets you add a new item to the site without a total redesign.