Though I'm not a Platinum, I can also add on that SPG also has SPG Moments Auction and SPG Instant Awards.

SPG Moments allows you to bid/buy special events and such using points.

SPG Instant Awards can be used to redeem for certain amenities/services at the respective properties. Every property have their own list of Instant Awards menu, some are abundant while some are scarce.

Some posters have also mentioned the conversion to miles. So SPG points have a variety of uses besides the usual room redemptions which makes the currency valuable.