Better box model by default. Everything in Bootstrap gets box-sizing: border-box, making for easier sizing options and an enhanced grid system.
Personal i think most profits are for the grid(s). In Twitter's Bootstrap all grid are fluid. Columns are defined as percentage of the total. But the gutter have a fixed width in pixels. By default an padding of 15px on both side of the column. The combination of width in pixels and percentage could be complex. In the border-box this calculating is easy cause: the border-box value (as opposed to the content-box default) makes the final rendered box the declared width, and any border and padding cut inside the box.