The Merganser is a synthetic overbag that can go over a down sleeping bag. Using a synthetic overbag is sometimes a necessary way to keep your down dry in really cold, or really wet environments. In really cold environments the moisture that your body puts out in the night will travel through your sleep system and condense wherever the dew point is, which is typically inside of your down loft somewhere. Adding a Merganser Overbag to the outside pushes that dew point out of the down and into the Climashield Apex batting. From here you can remove the moisture from your down and pack it away dry. The moist Merganser can either be dried during the day, or left moist and packed separately. If the moist Merganser is reinstalled over the down bag the next night your body temperature will continue to push the dew point into the Climashield and the down bag will continue to stay dry. The Merganser will continue to accumulate moisture and will need to be dealt with at some point, but Climashield APEX is remarkably good at dealing with moisture. It does not lose loft and dries very fast.
In really moist environments above freezing, the moisture is likely to come from the outside. A Merganser Overbag is not waterproof, but it can catch light moisture that comes in the form of condensation or contact with wet tent/tarp walls or whatever. This moisture will soak into the Merganser and need to be dealt with at some point, but it creates a barrier between the moisture and your down system.
It also doubles as an insulated poncho. This can allow you to either leave a jacket at home, or at least bring lighter insulating layers. This can often fit over a small backpack too, keeping things somewhat warm back there.