It's not a "discount" it's a rebate which appears in the form of Reward Cash which you can then use to pay off some of your credit card bill or to buy other things. At the moment, for example, I use RC to buy $50 FoodPanda coupons for RC45, thereby effectively getting a further $5 off each FP order (in addition to the 4% new RC which I earn by using the Red card for the order).
Note also that Red Hot Rewards of Your Choice applies to all HSBC personal cards except the Red and EveryMile cards, so that includes Premier Mastercard, Gold Visa card, Platinum Visa Card, Visa Signature card. The Visa Signature card also comes with an extra 1.2% on the first $100K of annual spending in all RHRYC categories in addition to the further 2% on the chosed RHRYC category.
I'm surprised you use a Premier Mastercard much - I've had one for many years but very, very rarely use it. It earns 0.4% or 2.4% on your chose RHRYC category. There are much better options available.