Where you place items in your oven depends on what the recipe recommends and where the heating elements are located. Baking a cake, muffins or cookies too close to the heating element may cause uneven baking and burning in some cases. Many recipes recommend placing your baking pan in the center, as this allows heat to approach it evenly from all sides.
Many electric or gas ranges have dual heating elements, with a baking element located in the bottom and a broiling unit located in the top, or ceiling, of the oven. In some cases, only the bottom element comes on for baking purposes, distributing the heat only from the bottom. Placing a baking pan halfway up, or on the middle shelf, away from the bottom element assures that the bottom of the cake, pie or muffins won't over-bake or burn before the center and top are done, as it allows plenty of space for the hot air to circulate.
A convection oven may have one or two burners, but it is also equipped with a fan that circulates the hot air evenly around the inside of the oven. This enables you to use all the shelves supplied with the oven at once, as there are no areas that get hotter than others.