I don't understand

Quoting from https://forum.grin.mw/t/next-pow-cuckaroom-unveiled-at-grincon1

“Grin’s unique dual PoW system allows for a smooth transition from GPU
mining to ASIC mining, with guaranteed rewards for the secondary PoW
where GPUs need not compete with ASICs.”

So it’s not left over scraps, but a guaranteed share of rewards (currently 48%). And until ASICs appear (which could be months away), GPUs can also enjoy the rewards of the primary algorithm…