Mostly when you look at Responsive design boilerplate stuff, they use grids. The reason behind that is that most of these templates are designed to work with a variety of content and layouts and in order for the pre-designed code to work/scale/function properly, there has to be some inherent organization. I think another reason is that it is generally considered good practice to design for a grid, and so it has been integrated into these boilerplates.

That said, i downloaded this http://cssgrid.net/ and looked at the code for a bit to see how it worked, then used that to start from scratch.