Circle casting... It is mainly used in Rites or powerful spells. The reason is to seal off the energy from the outside of the circle and from the inside of the circle. Like if your doing a spell to talk to the gods or goddesses, you don't want bad energy or - energy to entry into the spell. You also don't want if your doing some Black Arts spell you don't want that bad energy getting all over your stuff that is not part of the spell.
The point it, it is used to protect you and other things from energy. It is best used when doing Rites, Black Arts (Have to do it if the spell is a Black Art spell), Summoning (Again have to do it), or any spell you think that needs to be sealed until finished with.
Hopefully this helps.