Slowly changing fact tables provide a complete history of changes for some part of the business. Typically, such fact tables are used for situations where a relatively small number of changes occur over time, but we need to track all of these changes.
A good example is an inventory stock holding that does not change very often. In this case, a slowly changing fact table can record both the current inventory levels as well as providing a means of seeing the state of inventory holdings at any point in time.