Some people would prefer to use a theme that has a familiar backing to them. I think a lot of themes boil down to mostly using the grid system of the framework, but its nice to at least have familiar/standard grid to work off of once you've purchased a theme. Some people also prefer the visuals of some of these frameworks so allowing them to use the framework of their preference when you choose to sell a theme is a good reason to use something like this. Other elements and JavaScript elements like the modals and things are a nice bonus you get from theming a well-built framework like Foundation.