Aluminium foil is a very useful, flat, and malleable material, perfect for making impromptu roasting racks, pie crust shields, and simple food wraps (for non-acidic foods). Lining pans with foil can help speed up the cleaning process — just toss the foil in the garbage — but there is one thing in your kitchen that you should never line with foil, no matter how messy you think it might get: the floor of your oven.
Even Reynolds (aka “Big Aluminium”) thinks this is a bad idea, and discourages such activity on its website:
To avoid possible heat damage to your oven, we do not recommend using aluminium foil to line the bottom of your oven. Rather, we recommend that you place a sheet of heavy duty aluminium foil on the oven rack beneath the pie or casserole you are baking. The foil should be only a few inches larger than the baking pan to allow for proper heat circulation. The foil will catch any drips before they reach the oven bottom.
Ignoring this advice would not be wise. Placing foil on the floor of your oven not only blocks the heat from circulating properly, it can reflect heat back onto the heating elements, potentially damaging them. If you have a gas oven, the foil can block or otherwise interfere with the flame.
If you’re worried about drips and the like, you can place a sheet of heavy duty foil directly on the rack under your bubbling casserole or braise. You can also use a sheet pan (though you will have to clean the sheet pan). Whatever drip-catching device you settle on, just make sure it doesn’t take up the entire rack — you need that hot air to flow around your food to cook it, which is, after all, the whole point of an oven. (If any drips make it past your foil, you can always clean your oven.)