Not completely sure if that was serious or sarcastic, but in case that was a real question: The main reason for using a grid is to make sure elements on your page stay aligned vertically and equally distributed horizontally.

If you use separate margins and paddings for each element, there's a bigger risk that you'll mess up and introduce irregularities in the layout.

Of course, maybe you don't want elements to be aligned on your page. In that case, yes, you might not need a grid.