Cover a dish with oiled aluminum foil?

Here’s a good-looking recipe for vegetable lasagne.

The recipe is designed to be baked ahead and frozen. The instructions for heating begin: “To Bake From Frozen: Remove plastic wrap; cover baking dish with lightly oiled aluminum foil, and place on a large rimmed baking sheet.”

Oiled aluminum foil? What’s that about? And which side is supposed to be oiled? I never heard of this before.