because he knew that mikami would write takada's name in his death note so that giovanni would have access to the real death note to replace it. Therefore when mikami went to the wharehouse to write down the names of the spk and japanese taskforce he would be writing it in a fake note so that kira would fail. If not for this the death note wuldnt have been replaced by a fake and kira would have won. Also he did it to prove near could not surpass L on his own as near did not originally consider a fake note.