There are many laws, many rules in this Craft. Even though it is a Craft of freewill (an it harm none, do as thou wilt) there are consequences to all actions. The trick is to make sure you're always protected. This means minding the Rule of Three and Harming None.
Casting a circle is useful for two main reasons. One, it gives you a protective shield from any negative energy or outside forces. Two, it confines your energy space so it is easier to work. When the circle is open it is not broken. You have simply released the power back into the Earth.
No, casting a circle can be as simple as 1,2,3.
An important thing to remember is that your circle isn't really a flat circle. Imagine a SPHERE, totally enclosing you inside of it. Even though you may mark, draw or trace a flat 2 dimensional circle�it is much deeper than that.
Hope this helps for now. Remember, we're all learning!