Why separate the ground bar from the neutral bar in a sub-panel?

In a main panel, the neutral bar and the ground bar are bonded together. This means that the neutral wire and the ground wire are connected at the same point. However, in a sub-panel, the neutral bar and the ground bar must be separated. This is because the neutral wire is a current-carrying conductor, while the ground wire is not. If the neutral and ground were bonded together in a sub-panel, it would create a parallel path for the neutral current to flow on the ground wire. This could be dangerous, as it could energize the metal parts of the electrical system that are connected to the ground wire.