All regulatros change pressure in response to throttle movements.

On an N/A car, the pressure needs to be compensated for due to differences in the inlet manifold. When the throttle is closed, there is a vacuum in the manifold so the fuel pressure must be reduced. When you open the throttle, the vacuum decreases until it's nearly at atmospheric pressure so the pressure must increase to compensate.

On a turbo car, the same principal is applied but once the turbo comes on boost, the manifold pressure goes above atmospheric and the fuel pressure must rise to compensate again. Regulators are usually rising rate at a 1 to 1 ratio. For every bar of boost, the fuel pressure must rise a bar of boost.

The reason why you need to compensate for changes in the manifold pressure is because of the pressure differential. If you have 3.5 bar of fuel pressure without the vacuum pipe connected on the regulator (atmospheric pressure) and then came on boost at 2 bar, there would only be a differential pressure between the manifold pressure and the fuel pressure of 1.5 bar. This means that you essentialy only have 1.5 bar of fuel pressure and your flow decreases also. if you fitted the vacuum pipe back on, it would probably drop to 3 bar on idle as there is now vacuum in the manifold. As you come on boost, the regulator sees boost pressure and increases the fuel pressure to compensate. Instead of 3 bar as it was at idle, (or 3.5 bar at atmospheric) it will now be 5.5 bar at 2 bar of boost. Although it's now at 5.5 bar of fuel pressure, the pressure differential in the manifold to the fuel rail is still only 3.5 bar as you have 2 bar of pressure inside the manifold and 5.5 in the fuel rail.

The regulator is there to maintain a constant fuel pressure for the ever changing pressure differences within the inlet manifold.