In Wicca, the altar is used as a focus, as a sacred place. Physically it holds the tools that the witch will be using during the ritual and can be decorated as a symbol of why the ritual is being performed. Its presentation can also enhance the atmosphere within the circle itself. In essence, the altar can be viewed as both a physical and a spiritual offering to the Gods.
As such, the altar is treated with reverence. It not only holds symbols that represent and honour the Gods, it also holds symbols and essences of the five elements that surround us and are within us. The symbols of the Gods are there as a sign of respect and to remind us that all actions performed within our sacred space are in honour of Them. Wiccans believe that the divine is within everything, including themselves. An altar dedicated to the Gods is then part of the Divine, but the Gods themselves do not reside only within the altar.
There are many traditions and forms of Wicca, practiced both by groups and by solitaries. The placement of the altar can depend on the teachings of the particular tradition or it can be just what feels right to those who are setting up their sacred space. Generally an altar will be placed either in the centre of the Circle or in one of the Quarters or Cardinal Points of the Circle.
There is no one correct place for the altar within a circle. It is the intent and understanding of that placement, by all involved, that matters. Mostly the altar is positioned according to esoteric concepts but sometimes it is for a purely physical reason. There may be uneven ground or large piece of furniture in the spot usually reserved for an altar. There may be a fire-pit in the centre of a circular area that is perfect for the ritual, but would, of course, make a central altar unfeasible.