Data-Party contains the rules or behaviors that tell the inheriting class how to act. Data-party is an entity object relationship, meaning the object becomes an embedded object in pyWorkPage (case or work object) that represents a person, operator, company, government agency or organization. If it didn't inherit from Data-Party the object would not behave as expected/needed.
Our case inherits from Work-Cover- so it behaves as a case object and can be processed through a life cycle.
Pega has many relationships defined in a similar way...Log-, Rule- and so on.