An integral component to a layering system, it's primarily for increased warmth over a base layer or base+breathable insulating layer. Today's DWR finishes are generally effective against mist and drizzle but not full-on rain and while in theory a WPB jacket does the same thing in practice I have yet to use one that doesn't become a sweatbox while hiking.
As the name implies, they're especially effective preventing (convective) heatloss from wind. e.g., cycling over a mountain pass you generally arrive hot and sweaty then face miles of downhill at much higher speeds and a much lower work rate, perfect conditions for hypothermia. Donning a shell at the top is SOP.
Hiking doesn't have such dramatic transitions unless you're popping out of treeline into a windy alpine zone. But a simple rest stop can also bring a fast chill and 3 oz in you exterior pack pocket can make a huge difference.
Edited by Rick_D (09/24/20 12:22 AM)