padding and margin are added to the overall width. To avoid having to change with width setting every time you adjust padding/margin, it's easier to have an inner div on which you place the padding/margin. You just don't set a width on that inner div, in which case it just absorbs the padding/margin.
Really, though, in a situation like the one on that page, it never looks very good (to me, at lest) to have a background color on that content div. So it's a problem you can do without. (And I presume pink on green is for testing porpoises only? Surely? ...)