One reason that I have had to fax receipts in the past
is because the hotel has taken my husband's name (or in the case of a Le Meridien stay, my 9 year old son's name), which they get from the passports, and placed it on the folio. Even though my spg number appeared on the folio, and on the reservation, the hotel didn't credit me because the name on the folio didn't match the name on my SPG account. By faxing the receipt I was able to allow them to identify the correct folio.
I am always irritated when it happens but I understand the confusion, especially in a foreign country.
I now just scan the receipt and send a pdf via email. Prevents me from having to remember how to use the dusty fax relic all the way in the other room.