If you want to understand how the BOV works, imagine a piston in cylinder with springs on each side. When the throttle plate slams shut, the intake manifold goes into vac, while the pressure spikes in the piping (which causes compressor surge). The combined pressure of inside the piping pushes the piston in the BOV up some, but it will not fully open without the vac from the intake manifold pulling on the other side. If the BOV works off of just the pressure spike in the piping, it is probably open at idle also. When both the piping and intake manifold are under equal boost, the BOV stays centered with even pressure on both sides.
The boost spike in the piping will also help hold the WG open while the BOV is open, to route the exhaust gasses around the turbine and let it slow down more quickly. Having the WG on vac from the manifold will hold the WG shut harder as soon as the throttle closes, which is less than ideal.