The Mission abstract class defines the execute method, which calls the other instance methods in a particular order. You'll notice that the methods called are not defined by the Mission class. This implementation detail is the responsibility of the extending classes. This use of abstract classes allows child classes to be used by code that takes advantage of the interface defined by the abstract class.

In the template function pattern, it is the responsibility of the child classes to define the steps. When they are instantiated, and the execute method is called, those steps are then performed in the specified order.

Ideally, we'd be able to ensure that Mission.execute was not overridden by any inheriting classes. Overriding this method works against the pattern and breaks the contract associated with it.

This pattern is useful for organizing data-processing pipelines. The guarantee that these steps will occur in a given order means that, if side effects are eliminated, the instances can be organized more flexibly. The implementing class can then organize these steps in the best possible way.