Long before circles were adopted by ceremonial magicians, they have appeased in folk tradition as people for thousands of years have danced in circles around a festival fire. They offer power as well as protection.
In essence you can cast a circle anywhere for any kind of spell or ritual (a visualized one using your index finger takes seconds). If someone is being spiteful at work, you can swirl round in a circle in your chair casting a circle close to your aura (about an extended arm span all round) so that nasty words bounce off or are diverted back to the speaker.
Any circle you create should enclose yourself; your altar and tools, if you are using them and anyone working with you to create an enclosed protected place of concentrated power. Even if you are carrying out a whole spell with words or in your mind you can cast a visualized circle of light.