I just found a very interesting blog written by Dave Green (an architect of the Windows Workflow Foundation). One of the posts explains the main motivations for creating a workflow framework.

In general I am a big fan of the work this team has done. I believe WWF will become the main application model for serous service oriented applications; just like WinForms and WPF (Avalon) are application models for client application and ASP.NET is the application model for web applications.