Why is Abstract class useful?

Answer 1

In OOP abstract classes are useful to implies what the class need to do, but without implementation (how to do). This enforce a class to follow the rule, in our example an Enemy need to attack, without the abstraction you can create an Enemy derivation without an attack, the pure virtual method force an implementation. That contributes to make good polymorphic classes.

Answer 2

look at the last text in the abstract class section, it explains it