So why do you do it?
I know you are all thinking this, and I am just now getting to it. The reason is much simpler than you might think. Protection. That is one of the top reasons why you do something! You cast a circle so that it will filter out the negative thoughts and energies from, and around, you before spell work. Here is what the Gardnerian Book of Shadows has to say:
(Many thanks to www.hermetics.org/pdf/bookofshadows.pdf for providing the Book of Shadows)
"It is most convenient to mark the circle with chalk, paint or otherwise, to
show where it is; but marks on the carpet may be utilized. Furniture may be
placed to indicate the bounds. The only circle that matters is
the one drawn before every ceremony with either a duly consecrated Magic Sword or an Athame. The circle is usually nine feet in diameter, unless made for some very special purpose. There are two outer circles, each six inches apart, so the third circle has a diameter of eleven feet.
 Having chosen a place proper, take the sickle or scimitar of Art or a Witch's Athame, if thou mayest obtain it, and stick it into the center, then take a cord, and 'twere well to use the Cable Tow for this, and loop it over the Instrument, four and one half feet, and so trace out the circumference of the circle, which must be traced either with the Sword, or the knife with the black hilt, or it be of little avail, but ever leave open a door towards the North. Make in all 3 circles, one within the other , and write names of power between these.
 First draw circle with Magic Sword or Athame.
 Consecrate Salt and Water: Touch water with Athame, saying, "I exorcise thee, O creature of Water, that thou cast out from Thee all the impurities and uncleannesses of the Spirits of the World of Phantasm, so they may harm me not, in the names of Aradia and Cernunnos."
 Touching Salt with Athame, say, "The Blessings of Aradia and Cernunnos be upon this creature of Salt, and let all malignity and hindrance
be cast forth hence from, and let all good enter herein, for
without Thee man cannot live, wherefore I bless thee and invoke thee, that thou mayest aid me."
 Then put the Salt into the water.
 Sprinkle with exorcised water.
 Light candles; say, "I exorcise thee, O Creature of Fire, that every kind of Phantasm may retire from thee, and be unable to harm or deceive in any way, in the names of Aradia and Cernunnos."
 Caution initiate (if any); warn companions; enter circle and close doors with 3 pentagrams.
 Proclaim object of working
 Circumambulate 3 times or more before commencing work.
 Summon: "I summon, stir, and Call thee up, thou Mighty Ones of the East, South, West, and North." Salute and draw pentacle with Magic Sword or Athame, the first stroke being from the top down to the left."
So once again it explains HOW to cast a circle, but not WHY you cast a circle. The Alexandrian Book of Shadows similarly has no explanation as to WHY. The Rede of the Wicce is our only hope here:
Cast the circle thrice about,
To keep the evil spirits out
So from this I can say that we are constantly attacked by evil spirits during our rituals? I think not.
Well that's only partly true...With great power comes responsibility. If we wield great power then, yes, we will probably be attacked by evil spirits, because they want the power for themselves, just like it is not a good idea to meet people on the Astral Plane without knowing them in real life. Some evil spirits have fun torturing beginners. I know its sad, but it is really true.
So I have established that you cast your circle for protection correct? OK good. Now I want to talk about the different ways you can cast a circle .
I know many of you Eclectics out there were looking at the expert from the Gardnerian Book of Shadows, and thinking, that is way too to complicated when you could just use some much simpler ones. And many of you beginners were thinking...circumumbulate is not a word (and don't worry, my FireFox browser designates it as a misspelling), don't worry there are many many other ways to cast a circle.
One I found from Silver Ravenwolf, was:
"I conjure thee o great circle of healing so that you will be for me a boundary between good health and disease, a healing place of perfect love , trust, peace, joy, containing positive healing energies I will rise herein."
(many thanks to Silver Ravenwolf!)
So this is a great circle casting for...HEALING work...but what do we do if we are not doing healing work? Well, some people like to summon the five elements (Water, Fire, Earth, Air, and Spirit). Some people like to do this by calling the spirits of the 5 elements through their tools, Dagger for Air, Chalice for Water, Candles for Fire, Pentacle for Earth, and lighting Incense or something like that for Spirit. What I like to do is sprinkle water (for water), salt (for earth), and incense ashes (fire), then I like to light incense and walk around the circle, then also sit in the middle and imagine my circle transforming into a temple for the Gods and welcoming them in. Some other people like to just sit in the centre of the circle and meditate and imagine a cone of power appearing around them or something of the sort.
So these are all great ways to cast a circle, and I hope I have helped you understand why Wiccans cast the circle. I would like to just throw in that the Ceremonial Magicians actually cast circles too, its just that theirs is a bit different...though I don't know much about how they do it...so I will leave you to your own opinions, and research on that one!