I use this in conjunction to the Planning by Month document also listed (free). I look at the activities I have planned, materials needed and printables to print-and decide what I need to prep.
I also glance over the Planning page as well to see what materials I have and which I don't. If I don't have something, I add it to this list. Our family works from a budget, so I try to break things down over the course of the month that I will purchase. This helps me to see everything at large that I will need and purchase in advance, instead of having to buy everything last minute.