Why would people use Grab Pay rather than using a credit card to directly pay for stuff?

If you are into the Grab ecosystem, e.g. take grab rides, food delivery, then it may be an useful consideration. This is because you can exchange for various perks from the grab rewards points.

If you are not into the Grab ecosystem, then it is probably not worth its cause.

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Grabpay gives you points.

For some big-ticket items like dentist and etc. 1000$ treatment = 10000 points. That would be a 20$ off grab ride.

Credit card miles are not as attractive and cash back usually got capped or clauses.

Most solutions offered would involve topping up GrabPay using a credit card followed by checking out with GrabPay /FavePay at the merchant. This triple dipping is however limited to only GrabPay or FavePay.
I present to you a solution that allows you to maximise the 10%(5%+5% for Grab) cashback by using UOB One Card to top up GrabPay. First, top-up GrabPay using your UOB One Card. Next, to circumvent the GrabPay/FavePay limitation, move your GrabPay credits to a multi-currency card like YouTrip or Revolut by topping up to these cards using GrabPay Mastercard. Now you can spend your GrabPay credits through a contactless payment channel like Paywave through Youtrip or Revolut. This applies to overseas spending too as both Youtrip and Revolut support competitive wholesale FX exchange.
Regardless of whether the merchant accepts Grabpay, Favepay or just simply Visa(Paywave)/Mastercard(Paypass), you're able to MINIMALLY realise the 10% cashback. For Grabpay and Favepay, you'll enjoy the benefits of triple dipping. For the rest, you'll still get 10% cashback.

For people who want to earn credit card rewards without paying currency exchange fees, you use your credit card to top up your grab pay and use GPMC to top up Youtrip/Revolut for overseas use. Hence, you earn credit card rewards overseas without paying any fees.