i've benefited most of my life from IATA discount rates because my dad has an IATA card due to him working in a travel related organization even though he is an accountant there. Every time we travel together we get really cheap rates. $40 at the hyatt, $15 at accent inns in vancouver, free nights at the fairmont. benefits are great.
someone mentioned about checking the card upon checkin-- in my experience it only happens half the time. most of the time they don't check
as others have stated, the IATA field on hotel sites are usually for commission. If say my dad booked a room for someone and entered his # his company would get commission on that booking.