Why

Perhaps I haven't adequately absorbed previous information given about how a boat will act without the sails up, but I find myself wanting to know why the method chosen in each wind scenario is optimal. The instruction tells us what to do in each case, but not why. Also, I'm not clear on how a headsail and the mainsail react to wind differently, if in fact they do. Sorry to ask this if it's just a matter of my reviewing previous material.