A circle is a group of people you trust. It is an invitation-only club that you can invite or be invited into. When you get invited and join the circle, you can see your inviter and other circle members automatically. Circle members can invite other people into the circle as well, unless you as a circle administrator, barred them from doing so.
When you send a message within a circle, no email can leak outside by an accident. This builds ethical walls that prevent data and personal information leaks.
When you receive a message in a circle, you can be assured that the message comes from a circle member. This means no spam, no scam, and no ransomware.
The purpose of the circle in the organisation is to prevent accidental leaks and phishing attempts from the outside.