You might be wondering what a role is for?
In fact, the notion of "role" is present when carrying out many professional or private activities.
For example, depending on whether you are a client or an employee of a bank, you would not take your place on the same side of the counter: only employees have access to the “professional" part of the bank’s premises. It is also likely that the employee timetable is different from the bank’s opening hours for its clients, etc.
Many similar situations can be used to illustrate what a role is, and what is for. The diagram below summarises the details.