You would use the term when you think it would be in the be in the best interest of the party/s or person/s confronting you to leave and/or leave you alone. It should be stated with feeling.
Basically, "step off" is a form of brush-off, and/or an order to another individual to get out of your sight. You don't want them talking to you, or even near you. You want nothing to do with them.