For example INotifyPropertyChanged which is used in WPF.

The typical implementation is something like this:

     public string CustomerName  
        {  
            get  
            {  
                return this.customerNameValue;  
            }  

            set  
            {  
                if (value != this.customerNameValue)  
                {  
                    this.customerNameValue = value;  
                    NotifyPropertyChanged();  
                }  
            }  
        }  

When you have 20 properties you have a ton of repetitive code. With a preprocessor I could reduce each property to a one liner. You can also do tricks with reflection but that's way more complex to implement.