They serve two completely different functions...
The BOV opens during full throttle lifts which is why the internal spring is based on idle vacuum. When you lift off the throttle, the throttle plates suddenly close and the air pressure has nowhere to go. The resulting backpressure to the turbo can cause compressor surge, compressor backspin and turbo stalling which can damage the turbo. The BOV helps relieve backpressure which reduces engine stress, prolongs the life of the tubo and even shortens boost recovery time when you open the throttle again. But one thing it does NOT do is prevent over-boosting.
That's where a relief valve comes into the picture because it prevents over-boosting. It functions like a wastegate so it's similar to having a second wastegate, only this one is mounted to the plenum. The spring in the Greddy Relief Valve is adjustable and is normally set 1-2 psi above the setting on your wastegate. Should the pressure in your plenum exceed the setting on the relief valve, it will open and allow the excess pressure to escape. A wastegate should be sufficient but there can always be a problem, like boost creep.
Hope I explained this okay.