Why is “Self” assignable in Delphi?

This code in a GUI application compiles and runs (tested with Delphi 6 and 2009):

procedure TForm1.Button1Click(Sender: TObject);
begin
Self := TForm1.Create(Owner);
end;

Questions that come to mind:

• why is Self writeable and not read-only?
• in which situations could this be useful?

If you know the answer, I would be happy if you post it here or at stackoverflow.