Casting a circle comes from new age practices, ceremonial religious-based magick, and a few Middle Eastern traditions.
Those that say they cast a circle "for protection" have clearly never been involved in real magick and have never had any real interactions with the spiritual world. If a spirit, energy, or other person's spellwork is on the hunt for you, a silly little circle isn't going to stop it.
A circle unquestionably does not function as a filter - letting in "good energies" and filtering out the "bad energies." Substantive Witches and occultists chuckle at this new age notion. Witchcraft is NOT an open invitation to all entities and spirits, but rather, it is a step into the spiritual world. You must enter it feeling comfortable and confident, just as you would stepping into the physical world.
Old World Witchcraft focuses more on relationships with your altar space, herbs, materials, and talismans. Part of this practice involves routine cleansing of your space and sustaining constant spiritual protection - inside and outside of spellwork.
Take time to regularly smudge your space. Physically purify your home and cleanse your altar. Keep healthy plants (either living or cut) in your dwelling. Cleanliness is next to godliness. *wink*
Can you still create a beautiful candle-lined circle around your ritual space? Absolutely! Do it for ambiance. Do it because it makes you feel good. And when you feel good, your magick becomes remarkably more powerful. After all, that is what it is all about!
Love & Magick,