SATA= Slow (Max 6 Gbps) PCIe = Fast (Max > 100 Gbps in theory)
This is the maximum rate from the drive to the motherboard. Many drives are fast enough that SATA works become the bottleneck. With PCIe, the drive can run at its full speed, whatever that may be.
This is the mental equivalent of Saitama’s workout from One Punch Man: 100 sit-ups, 100 pushups, 100 squats, and a 10-km run. (Repeat daily until your hair falls out.)