In certain situations, with out the use of a bivy, my Gore Dryloft (GDL) shell on my Puma has provided protection from moisture.

Sleeping in an enclosed single wall tent like a Betamid or Silshelter, where condensation forms on the interior (and before it freezes), the GDL keeps the down dry should my bag be touching the sides of the Betamid.

When sleeping under a tarp in stormy weather, invariably there will be spindrift snow blow in the ends - assuming of course there is snow on the ground or on the trees above and the ends of your tarp are open. Snow blows off the trees and swirls around, even in well protected forested areas. This situation is even more pronounced if there is actually snow falling during a storm. In this situation the GDL, again adds that extra level of protection over a simple DWR shell.

In milder conditions the GDL on my MEC Emperor Penguin overbag works in the same manner with my first example above. When under a tarp in the rain, it certainly adds some protection from splashes of rain that find there way into the ends or under the sides of an open tarp.