As most of you know, the Magic Circle, or Circle of the Art, is a common instrument used in many of Western Esotericism. There are all kind of myths about what it is, and why it should never be broken, and also, it's uses.
So I will just give my own personal opinion about this particular topic.
First of all, a circle, as a geometric figure, separate the "All" of a "Part", now, the All is much, much bigger than the Part.
So we can assume a Circle is used to separate what is "Outside" and what is "Inside".
This little and almost silly explanation, while simplistic, it can be quite profound.
When someone cast a circle, by means of a Pentagram Ritual (in Golden Dawn traditions, or even Wicca), the Operator creates a miniature universe, where his or her Law rules. He/She does this by giving up the control over outside forces, and it's exactly why is actually quite dangerous breaking a circle. No matter how powerful and skilled the Operator is, once the Circle is broken, he or she will be defenseless against outside forces.
Of course, this is an overstatement, but it does make a valid point.
While there are many designs of magical circles in many grimoires, the True Circle of the Art is in the Imagination, the window to the Astral (Inner) Plane
In truth, the Circle is traced limiting the Sphere of Sensation (our Aura, as most people outside the Golden Dawn tradition call it), and it moved with us, it's a mean to make our Sphere of Sensation brighter and more powerful.
(Sorry, I know I usually post things a bit more...comprehensible, but there's a heat wave and I've about to faint
Hope you enjoyed this