谁能帮忙解释一下一个小问题

procedure TForm1.Button1Click(Sender: TObject);
var
  fullscreen:TBitmap;
  fullscreencanvas:TCanvas;
  dc:HDC;//
begin
  fullscreen:=TBitmap.Create;
  fullscreen.Width:=Screen.Width;
  fullscreen.Height:=Screen.Height;
  dc:=GetDC(0);
  fullscreencanvas:=TCanvas.Create;
  fullscreencanvas.Handle:=dc;
  fullscreen.Canvas.CopyRect(Rect(0,0,Screen.Width,Screen.Height),fullscreencanvas,Rect(0,0,Screen.Width,Screen.Height));
  ReleaseDC(0,dc);
  image1.Picture.Bitmap:=fullscreen;
end;以上程序很简单就是将当前windows屏幕复制到image上 但是有几个地方不明白
分别是
  dc:HDC;
dc:=GetDC(0);
fullscreencanvas.Handle:=dc;
ReleaseDC(0,dc);以上这些操作应该是跟句柄有关 为什么必须这么做 如果取消掉上面这些操作 会怎样？