Any circle you may perform magickally is a construct to keep your mind focused. It is not required to do any healing work, or workings in general.
When we do public ritual with moving parts and invocations, we move ritual participants in and out of the circle. In this specific case, some entry and exits will occur. In the ritual of Samhuinn, we invoke the Cailleach and she enters the circle and is ritually brought forward for the purpose of the Rite. In a poetic way, we drew the Cailleach from the energy in each of us.
If you reference modern thought on magick circles and magick in general, you’ll see advice not to get too hung up on constructs cause you’re just weakening your own magick capabilities by relying on them. Kerr Cuhulain in his book ‘Full Contact Magick’ is a great reference guide for more contemporary quantum thought on Magick and less 18th century ‘mechanicalism’. His book is very much in line with the OBOD traditional thoughts on the subject (druid and ovate grade) and expands a bit on areas of Magick that aren’t heavily outlined by the order.
I have a few hard rules I use for our public rituals. This is to keep them grounded well and understood by a wide range of participants that may or may not be druids: