A magic circle is commonly used in ritual work as a powerful symbolic tool.
The circle itself acts as a barrier between the microcosm and the macrocosm. As everything inside the circle is representative of the magician's microcosm, any symbols or names used within the circle bring about certain changes in the microcosm designed to enhance the effectiveness of the working.
While inside the circle, the magician is said to be in their seat of supreme power, giving authority over and protection from anything outside the circle that is within the magician's ability to handle. For this reason it is highly reccomended that a magician does not exit the circle during a ritual (especially demonic evocations), as this will make them vulnerable to attack.