Spring Web Flow(SWF) focus entirely on the definition and execution of complex conversation in a web application, i.e. web application page flow.
To understand why it is a good solution, it is a good idea to start with the problem it tries to solve and the complexities involved in the solution. An example case is the shopping transaction which involves adding/updating shopping cart, providing buyer info, confirming order payment, etc. It has a clear web page flow, if there is no security problem. However, there might be problems like, jumping payment, refreshing pages, etc. SWF is a good solution for those situations.
SWF can easily integrated with existing frameworks like Spring MVC, Struts, and JSF, in both servlet and portlet environments.