People tend to think that the reason for house wrap is to make sure that there are no air leaks. Well, yes, that is one reason. But another, perhaps even more important reason, is to ensure that the air you breathe is not coming in through all the construction materials, the siding and sheathing in particular. These materials contain harmful chemicals that you would not want to breath day in and day out. So the house wrap is actually there to keep your lungs healthy.
So how does the air come in from the outside, you ask? Well, when you open and close the door there is an exchange of air, but it is also a good idea to open a window for a couple minutes every day–even on a cold day. And there are also vents designed to recycle the air.