zabazor
...also keep in mind that the circle is not necessary, just helpful like all of the tools.
As I mentioned, it depends on the path. In some a circle is not only necessary, but critical... in others not so much.
In many of the paths were a circle is used/necessary, a circle isn't so much a tool, but an actual barrier between you/your circle mates, and the mundane world. It is a place between the worlds, where time and space operate a bit differently. Not only is it a way of keeping unwanted energies/entities out, but it is also a container that will how the energies that you raise during your ritual for use in your magical workings.
I've been in rituals where the circle was not properly cast, and can honestly say that of all the other "tools" that one might use during ritual, a circle really isn't something that you can do without (or use some other substitute for), as it definitely makes a difference in the ritual if the circle is missing (or improperly cast).