I was doing some insulation in the basement, and I noticed that the
previous owner had disconnected some ductwork (4") which connected to
the cold air return of the furnace. This ductwork lead to an outside
vent, and was intended to bring in outside air to the furnace.

I assume that since the builder went through the trouble to put that
in, that it's necessary.. but why is it necessary? Isn't there enough
air in the house for the furnace to use? The furnace has been running
for at least 4 years without it. I'm willing to repair this, but I'd
like to know the reasoning behind it. I've found out that vent is a
pretty major source of cold air coming into the basement.

Another bit of information: The house is very drafty, so there's no
concern about it being so sealed up that all the oxygen is gone. LOL