Why should our freezers be set to 0*F?

So it stays frozen all the way through.

When the freezer is set to 20 F, and you put something dense in there, (like a casserole) it will take a long time to thoroughly freeze. And while it is freezing, bacteria can grow, and when you cook it, voila! bacteria soup.

Same when you bring frozen foods home, if in the hour between pulling them out of the frozen food section and getting them in the freezer at home, a bit can thaw, if the store had it at even 10 F.

You want to be sure all items are and remain frozen. Something with salt, or sugar, or any other foods may have different freezing points.

@kritiper brings up an important point. 32 Degrees F is the freezing point of fresh water, but not of everything.