These days it is quite rare for me to actually use a magic circle ... Jason points out it’s a minority, Western-European practice that ... took off in the late Renaissance among the ceremonial bourgeoisie. It’s practical application is comparatively rare on a global level. [In Peter J Carroll's] experience [magic circles] afford little protection.The only point I would actually argue with is that the practical application of a Magic Circle is comparatively rare on a global level. I'd wager that every magical practice makes use of circles in their art. Every magical working space I've seen, no matter what the tradition, includes circles somewhere on the altar, in the sacred space, or wherever the actual conjuration of spirits takes place.
I suspect he means that globally, you won't find a neo-pagan form of the Golden Dawn's LBRP being performed before every magical act, and he's totally right in that regard. However, every circle, whether it is intended for magic, marketing, or doodling has an occult meaning and purpose. Agrippa explains it thus:
"a Circle doth answer to Unity, and the number ten; for Unity is the Center, and circumference of all things; and the number ten being heaped together returns into a Unity from whence it had its beginning, being the end, and complements of all numbers. A circle is called an infinite line in which there is no Terminus a quo, nor Terminus ad quem, whose beginning and end is in every point, whence also a circular motion is called infinite, not according to time, but according to place; hence a circular being the largest and perfectest of all is judged to be the most fit for bindings and conjurations; Whence they who adjure evil spirits, are wont to environ themselves about with a circle." (Quoted from Esoteric Archives for instructional purposes.)See, when you conjure spirits, they can't get into the circle unless they're invited. They hit the circle, and begin going around and around it, looking for a point of entry. When it is empowered through evocation and invocation, evil spirits can't get through it at all. There is no point of failure they can breach.
As you can see, it makes sense that late Renaissance ceremonialists would begin incorporating the use of the Magic Circle in their rites. They were conjuring a lot of spirits to get ahead in life, and the kind of getting ahead they had in mind was based on exactly the kind of desires for material things that attract the Evil Daimons referenced in the Corpus Hermeticum. The Circle became more necessary to them as they turned from the kind of hands on crafty magic to conjure magic.
But when you look at the grimoires of the times of the Renaissance, the Magic circle pops up primarily in conjure magic. Practical, hands-on magic doesn't require a magic circle because if you are conjuring spirits in the rite, it's through the virtues of the herbs, minerals, or symbols you use to make the talisman, mojo bag, or whatever physical fetish you use. You mix the ingredients, let the spirits that are naturally in tune with the ingredients manifest, and then you speak to them directly. There's less chance of getting an "Evil Daimon" when you're working with the direct manifestations of the spirits themselves.
But when you're doing the kind of formal conjurations of the Great Work that I do, a magic circle is a major part of your Work. You're going through the heavens seeking initiations. You need an added level of protection from deluding spirits. It's difficult at first to discern between a spirit telling you what is true and real and a spirit telling you what you want to hear. You don't want to conjure up a spirit of Venus for initiation into the sphere that rules the engendering of existence and wind up getting a spirit of debauchery and decay. Hedonistic self-destruction can be fun for a while, but it sort of sucks when the liver and kidneys fail, or when it burns when you piss.
When you're doing the practical Work I do on behalf of clients, it's also important to use the Magic Circle. Accidentally cursing your clients is bad form. When I do practical Work, I go Above to the Macrocosm, and Work with the Intelligences of the spheres and their minions here below in the material realm. I Work with my brothers and sisters and cousins in the Holy Family of Emanations of the One. Picking up a cousin who isn't suited for the task at hand can be disastrous. The circle keeps out the spirits I don't want, and allows only the spirits I do want.
My magic circle is about six inches wide. It's burned into the top of a cigar box that I use to house the electronics that light up the sphere when I lay a metal talisman across the electrodes sticking out of the Box. My magic circle is the Table of Practice I use to conjure the spirits in all my magical Work on behalf of the clients, my material needs, and my spiritual goals. I made it once, and it continues to function, properly empowered. I trace out a circle of defense around myself as well. It takes about half a minute to do so. I use a spherical crystal to communicate with the spirits. Using a magic circle doesn't take very long at all.
Throughout the day, as magical needs arise, I don't do a formal circle. I use techniques that are combinations of mental, physical, and verbal directives to the forces of existence. I use the forces of the spheres I've been initiated into. But for the formal stuff, I stick with the circles.
I guess what I really want to say here is that a Circle is important. It might not be used all the time by all the people, but that doesn't make it optional, or unnecessary. It's not a thing that can just be dropped without consequence. Other forms of protection are useful and more effective than trying to form a full magic circle on the fly as you go about your daily activities, but for the Ceremonial Bourgeoisie like myself, your magical Work should probably include a circle.