Why is a vacuum line attached to a fuel pressure regulator on many port fuel injected engines?

A vacuum line is attached to a fuel pressure regulator on many port fuel-injected engines to help maintain the appropriate fuel pressure relative to the intake manifold pressure. When the engine is under load and the manifold vacuum decreases, the regulator compensates by allowing higher fuel pressure, ensuring that the fuel injectors deliver the correct amount of fuel for optimal combustion. This dynamic adjustment helps improve engine performance and efficiency across varying driving conditions.