Зачем нам нужны конструкторы

constructors are used to initialize fields of class.

That we can do using declaring methods inside a class. like:

class A
{
  public int a,b;

  public void initializeFields(int x, int y)
  {
    a = x; b = y;
  }
}

class MainClass
{
    static void Main()
    {
        A a = new A();
        a.initializeFields(10,20);
        Console.Read();
    }
}

Instead of writing above code, why we go for constructors ?? Like: 

class A
{
  public int a,b;

  public A(int x, int y)
  {
    a = x; b = y;
  }
}

class MainClass
{
    static void Main()
    {
        A a = new A(10,20);
        Console.Read();
    }
}

I want to know the ADVANTAGE , SIGNIFICANCE of using the later code ??

I want to know the EXACT reason