Task flows are a tool to help us think through the design before a feature is actually developed. They allow us to interject the user into the flow of the application and determine if the conceptual model agrees with the user model.
The Information Architect (IA) is generally the team member taking the lead on diagramming the user narrative. With the business requirements and user modeling taken into consideration, the IA determines the tasks needed for a specific feature and maps out how the user will accomplish the task within the application. The resulting diagram shows a series of actions the user must do in order to begin and end the task, which can be anything from uploading catalog data to generating reports to assigning rights and permissions to user accounts.