If you were to sit the tarp on the hammock ridge line, you would not need a tarp ridge line any longer than the hammock ridge line. However, as you raise the tarp above the hammock ridge line, you get a gap that would allow rain to get into the hammock. Also, as others have said, it is a comfort thing. If I wanted to get out of my hammock and move around a little or maybe cook a meal without getting wet, I would prefer a little more room.

Dave E