I grew up in Montreal and lived further North than that as well.
In the winter time we kept our fuel tanks full for a couple of reasons:
1) As has been mentioned, water will condense in the empty space. This is made worse by the large difference in temp in the winter. Water can then freeze and that locks up the fuel line.
2) When you live in an area where you could get stuck in snow, having a full tank of gas to run your engine longer for heat can save your life.
Once you get used to seeing 1/2 a tank (maybe 1/4) as the spot to fill-up, it becomes habit.
I actually checked up on #1 with a couple of friends who were mechanics. They all said the same, it isn't the first time more water condenses, it is cumulative and it hits harder in the winter time when the water freezes.
Michael