I am hoping to better understand the science here. When using liquid flash to seal the joint when bottom plate/mud sill connects to foundation and sheathing to 2 by, why does flashing need to go up the sheathing a couple inches when using ZIP board as well as down a couple inches over concrete? I know everyone does it this way, so there must be a reason, but would like to better understand. Why not just fill the joint and let zip board do it’s job? For concrete perhaps you don’t want top portion of foundation wet so water is less likely to wick up? 

Same question when flashing window rough opening. Why flash the outsides and not just insides.

Much appreciate you helping me understand the building science!