Workflow Foundation provide a way of describing the order of execution and dependent relationships between pieces of short- or long-running work. This work passes through the model from start to finish, and activities might be executed by people or by system functions.
When to use Workflow Foundation
When you are building Application that involves complex data flow and decision making (Like Approval) logic that are hard to manage in conventional ways.
SharePoint is very well known application that uses Workflow for Managing Content Approval.
When you need to build complex rule base Wizard kind of things
When you have very complex business rule processing (see case study section for details)