When to use
Request something from the user
A dialog may be used to request something from the user in order to continue a user-initiated process. The system may not be able to continue due to missing information or something that should be confirmed by the user first.
Notify the user
A dialog may also be used to notify (alert) the user about something that has to do with the ongoing task. For instance, this could be a system error or an important consequence of a user’s action.
Provide (non-essential) additional details about ongoing process
Sometimes non-essential additional information about an ongoing process has to be accessible to a user, without having to leave the current view. A dialog could display that addition information without cluttering the main view.