I will cast a circle or other shape for some kinds of magic. I don't use it for religion, and I'm afraid I 'lifted' the concept from the wiccish people I met when I left home. It was my first exposure to western magic and I loved how fast and efficient it was, compared to what I had grown up with.
I cast a small circle, more of a sphere, and I install guardians in a subjective sense, seeing them as parts of myself specifically chosen for the design of what I'm trying to do, so they can be any set of (to me) balanced concepts.
The reason I do it at all is because I see a spell as fragile in it's beginning, needing a safe space to become fully formed so it doesn't get blown apart (more drifted than blown - it's a nebulous thing) by distraction and the regular patterns of what's around. A spell is a new pattern I want to release, and I like to use the circle for it to become strong in it's own right before I send it out, so it can overcome or find the proper space among the rest of the patterns out there.
I'm not explaining it very well, because I am not given to self-reflection, but if I am aiming for a specific result it usually isn't one that would happen on its own, or I wouldn't need a spell in the first place. So it has to be strong enough, or 'mature' enough, to lead to that result as naturally as whatever 'would' happen on its own, so it needs to have a certain amount of strength and inertia behind it before it goes out. Having a blank space, cut off from all the other patterns out there, lets it grow to fill that space, so it is less likely to drift apart when released.
I guess a circle for me is a kind of spell womb - I cast it to create the spell and dissolve it when I feel the spell can compete with everything else drifting around out there. then I launch the spell towards it's target, so it is more of a directed thing than a drifting thing. Swimming towards my goal rather than depending on the current to float it there, in a way, so more able to overcome the 'what would happen if left alone' and impose the 'what I want'.
Hey, it works in my head.

unsung