Riser with PCI-E = Voltage regulator needed because PSU gives only 12V over 6-8pin pcie.
Riser without voltage regulator = molex or sata = PSU can regulate directly.
Only advantage when you need a lot of power over pcie (i.e. RX; 1050/Ti without powerconnector)
You are confusing a transformer with a voltage regulator. A voltage regulator is designed to maintain a constant voltage and prevent spikes. It does not step down current, that's what a transformer does.
https://en.wikipedia.org/wiki/Voltage_regulator