Wrapping potatoes in foil does not decrease the cooking time of a baked potato. While wrapping in aluminum foil will help keep a potato warm after cooking -- and this is one reason for the practice -- cooking a potato in aluminum foil results in a potato with a dense interior and soggy skin that can be the breeding ground for bacteria.
Increased Cooking Time
Wrapping a potato with aluminum foil does not decrease the cooking time at all -- in fact it increases the cooking time. This is because when wrapped with aluminum foil the oven must expend additional thermal energy to heat the aluminum foil wrapping on top of heating just the potato -- in fact, aluminum foil actually reflects heat and is an insulator.
Soggy Skin and Dense Interior
Cooking a potato wrapped in foil allows very little moisture to escape from the potato while it cooks. This results in a soft, wet, soggy skin and a dense interior that is much more similar to a boiled or steamed potato than a classic baked potato with a crisp skin and a light, fluffy interior.
Food Borne Illness
Another major issue with cooking potatoes wrapped in aluminum foil is that natural spores of Clostridium botulinum can survive baking process in the low oxygen environment created by the foil. This can increase the chance of contracting botulism, especially when the potatoes are stored at less than 140 degrees Fahrenheit.