Use this skill anytime you are dealing with other people. Charisma is how you socialize, ingratiate, negotiate, convince, debate, or argue. Use it to question others and gather information. Or use it to intimidate and get your way.
You can even use it against your fellow player characters. If there is a disagreement about what course of action to take, you can insist that it be decided by a Charisma roll. Highest roll wins, and the other players must follow your plan.