There's already an answer. By having a season where you can't grow, you give the soil time to build up nutrients once more from various sources, or you rotate out the crops that can't grow in the cold for the ones that can that have the side-effect of the above.
...Or something like that, the specifics elude me, I'm not a farmer. But I've heard a reason before.
Also, winter could actually be a season long memorial to the time of the Windigos.