Boost spike is a major issue with the VNT turbo.  Back pressure or excess fuel can cause the boost to rise faster than the turbo's vanes can react.  Spike and surge can cause the ECU to react by pulling out timing from the motor, this results in lower torgue output. By installing the Boostvalve as shown above this excess boost can be prevented.
The Boostvalve is not used to raise max boost but to control spike/surge and effectively give better control of the turbo's wastegate.