A thought on why we sometimes want to go widdershins, counter clockwise: Widdershins does not cancel energy, it disperses energies that are there. It awakens the earth energy. It stirs it up. So, if you had cast a standard Wiccan clockwise circle then reversing and going counterclockwise disperses the boundary.
In situations where you want to invite spirits of the other world to come forth, widdershins would be the way to go to set up the circle. You would sweep to cleanse before hand, not lay down salt because that keeps everything out. (Salt cancels psychic energy.) Moving counterclockwise, you have made a place between worlds where you can invite and meet with Other Realms, where you can meditate on the roots of the World Tree and the underworld. You push the already present energy to form a boundary or closed circuit. You aren't raising and channeling energy for a barrier. Keep that in mind.
Going deosil, clockwise and with the movement of the sun in the N hemisphere, increases energy so the channeled force you will draw upon and direct through your projecting arm with your intention, will create a boundary and barrier. It will contain and hold your psychic energy and, if unbroken, will stop outside psychic interference. It makes a 3-D sphere, like an energy-clean room.
Now i fully admit that i get confused. Do we invite our Deities and the elemental forces within the circle, or are they outside? If within, do they pass through the barrier we've just created? Or is Deity everywhere and thus cannot be excluded, but pays attention when invited? My own answer here is that I think of "invited" as being synonymous with "connected." It would mean that a god-form "attends" or witnesses through my connection to Deity. I'm not sure if it is the same, possibly similar, with elements. A circle is a physical force, this I know and have seen it visibly and in action.