what is the advantage of Singleton Design Pattern

Question

every one know how to write code for Singleton Design Pattern.say for example

public class Singleton  
{  
    // Private static object can access only inside the Emp class.  
    private static Singleton instance;  

    // Private empty constructor to restrict end use to deny creating the object.  
    private Singleton()  
    {  
    }  

    // A public property to access outside of the class to create an object.  
    public static Singleton Instance  
    {  
        get  
        {  
            if (instance == null)  
            {  
                instance = new Singleton();  
            }  
            return instance;  
        }  
    }  
}  

it is very clear that when we create a instance of any class many time the memory is allocated for each instance but in case of Singleton design pattern a single instance give the service for all calls.

1) i am bit confuse and really do nor realize that what are the reasons...that when one should go for Singleton Design Pattern. only for saving some memory or any other benefit out there.

2) suppose any single program can have many classes then which classes should follow the Singleton Design Pattern? what is the advantage of Singleton Design Pattern?

3 in real life apps when should one make any classes following Singleton Design Pattern? thanks

Here is thread safe singleton

public sealed class MultiThreadSingleton   
{   
    private static volatile MultiThreadSingleton instance;   
    private static object syncRoot = new Object();   

    private MultiThreadSingleton()   
    {   
    }   

    public static MultiThreadSingleton Instance   
    {   
        get   
        {   
            if (instance == null)   
            {   
                lock (syncRoot)   
                {   
                    if (instance == null)   
                    {   
                        instance = new MultiThreadSingleton();   
                    }   
                }   
            } 

        return instance;   
        }   
    }   
}

Answer 1

To assure only one and same instance of object every time.

Take a scenario, say for a Company application, there is only one CEO. If you want to create or access CEO object, you should return the same CEO object every time.

One more, after logging into an application, current user must return same object every time.

Answer 2

One useful place to use a singleton is if it is accessing some resource that you only want to have a single access point for. For example, I've used it when writing some code to talk to a device. I only want one piece of code talking to the device so I use a singleton. Any attempt to create another instance of the object that talks to the device will just give you the same object back so I never have to worry about two instances maintaining out-of-sync data about the device or getting messages to and from the device mixed up or out-of-order.

But, of course, you are under no obligation to use them. They are just a tool that is sometimes useful.