a circle is used to create sacred space for some. it is considered a space between the mundane and the magical and is believed to be the best place to perform magic.
to cast a circle you project energy through your wand (finger) and move clockwise from east to south to west to north to east. once the circle has been cast you can call the spirits of the elements from each of the directions.
That's correct, a circle can also be used as a protected area, or a summoning space, this is because summoning a being into a room may set free an unwanted visitor, but in a circle, they are unable to go into the outside world.
Casting a Circle is like erecting a temple or a church. It's meant to keep negative energy out and positive energy in. It's meant to protect you and the ritual.
There are many ways you can cast a circle! Casting a circle is done by ritual. There are many rituals on how to cast a circle! However, there is no one true ritual or a certain way (depending on your tradition) to cast one. Usually casting a circle requires salt and water. These are wonderful elements to represent cleansing. There is also an item such as a wand, athame, a stick or whatever you would like to use to direct your energy.
You can search on the internet circle casting rituals to get a good example on how one is done. Remember there is no true or wrong way to cast a circle. The main idea is to keep bad energy out and to protect yourself within the circle.
Is casting a circle necessary for doing ritual or spells? I do not know. It depends on YOU and how you perceive things!