Head First Design Patterns - Decorator Pattern using Starbuzz

I am going through the book Head First Design Patterns and am specifically looking at the Starbuzz example for the Decorator pattern.

I am having trouble understanding that what exactly is the need for CondimentDecorator in the example provided. Why can't Mocha simply extend Beverage? What's the need for another layer of abstraction?`

public abstract class Beverage  
{  
   String description = "Unknown beverage";

    public String getDescription()
   {
     return description; 
   }

   public abstract double cost();  
}  

public abstract class CondimentDecorator extends Beverage  
{  
    public abstract String getDescription();
}  

public class Mocha extends CondimentDecorator
{  
   Beverage b;

   public Mocha(Beverage b)
   {
     this.b=b;
   }

   public String getDescription()
   {
      return b.getDescription() + ", Mocha";
   }

   public double cost()
   {
     return .20 + b.cost();
   }
}