What is the use of smartfactory in Test Class in Salesforce ?

SmartFactory aims to provide test data objects with all required fields and object lookups pre-populated.
The idea came from writing unit tests for unfamiliar orgs, where setting up test data might mean digging through a large hierarchy of lookup relationships and required fields. Instead, SmartFactory uses the Describe metadata to populate all fields with data of the right type. For lookup fields, it creates an appropriate object and then uses that object's id.