The outer layer is your final shield against what nature is throwing at you.  It should protect you from rain, wind and snow.  Often, the outer layer is made from an uninsulated shell fabric as your insulation is taken care of with the middle layer(s).  In days of old, this is where the layering system could break down due to shell jackets that didn’t allow moisture and warm air to escape.  Thanks to the innovation of fabric technology, we now have a range of waterproof/water repellent breathable fabrics that can breathe, block wind and rain keeping the wearer far more comfortable than ever before, even during high output activities.