why the page is post back and inform to the server when the button click

namespace CustomControls 
{  
   public class MyButton: Control, IPostBackEventHandler 
   {     
      // Defines the Click event.
      public event EventHandler Click;
      
      // Invokes delegates registered with the Click event.
      protected virtual void OnClick(EventArgs e) 
      {     
         if (Click != null) 
         {
            Click(this, e);
         }  
      }
      
      // Method of IPostBackEventHandler that raises change events.
      public void RaisePostBackEvent(string eventArgument)
      {     
         OnClick(EventArgs.Empty);
      }
      
      protected override void Render(HtmlTextWriter output) 
      {     
         output.Write("<input type="submit" name=" + this.UniqueID + <br mode=" hold=" />            " value="Click Me" />"); 
      }
   }    
}