It's just a convention to make it easier to be consistent, simplify your markup, and make development easier. Having a grid with a few defined classes for fitt

It's just a convention to make it easier to be consistent, simplify your markup, and make development easier. Having a grid with a few defined classes for fitting into that grid is much simpler than building each page from the ground up. They also act as a shortcut for screen resolution compatibility. See http://lessframework.com/ http://lessframework.com/ for an example (there are other frameworks, but Less is just the one I remember right now).