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ProcessMessages interrupts the execution of an application so that Windows can respond to events. So, if you will remove Application.ProcessMessages(); string from this example, then you will never see 'Some text' string on caption of the form.

procedure TForm1.Button1Click(Sender: TObject);
begin
  Caption:='Some text';
  Application.ProcessMessages();
  Sleep(3000);
  Caption:='Form1';
end;