posted 9 years ago
The purpose of an abstract method is to provide a name (and signature) of a method which must be implemented by every concrete subclass of the abstract class. So if you are designing a type which must have a particular method to do its job, but you expect that subclasses of that type will have a variety of ways to implement that method, then that's when you might use an abstract method.
That may sound unclear to you, but if you're a beginner then you may never have designed a class in your life. Typically you're told to write a particular piece of code, and learning to design classes comes along later. And if you're not especially familiar with inheritance of classes, which also doesn't get taught to beginners right away, then you might well not understand why that's a useful design feature.