BigV's right, it's all about the moisture. A foil tent won't keep moisture trapped inside as well as a dutch oven (lidded cast iron pot, just to be sure I'm not using words you use differently,) but it's better than nothing. With a dutch oven, you wouldn't need to add water, with a foil tent you will. The top will still get nice and roasty, it won't appear to have been steamed. See
here for how I do it (but I do use a dutch oven.)