I'm learning about design patterns and in examples of code I've seen a convention where the abstract class declares a method, for example:

public abstract class ServiceBase {
... 

public virtual object GetSomething();

and then

protected abstract object DoGetSomething();

My question is on why these two methods exist, since they appear to serve the same purpose. Is this so that the base class GetSomething() method logic cannot be overridden by inherited classes? But then again, the method is marked virtual, so it can be overridden anyway. What is the usefulness here in requiring derived class implementers to implement the abstract method when the virtual method can be called anyway?