You are right about the face value being the largest in the world and also right about putting down your IC number whether you ask for on or deposit one in the bank.
It is a very convenient piece of paper if you need to smuggle money. Obviously such a big note is intended for this purpose as Singapore is in CIA's money laundering list. For businessmen like you need to bring in excess of the limit out of or into a country it is so convenient. As for the bank taking down your IC number now sweat they will not check if you get the note not by illegal means and you can explain, as its just for reference.
If you have a note like this you can get a good exchange rate overseas as they are always eying on this because they could sell at a better rate for those who really need it.