Unfortunately, it's a necessary evil, and we have to live with it. Here's why: You're under boost, the turbo is fully spooled, and life is good - then you shift. That means your foot comes off the gas - and the throttle plate slams shut. Suddenly, instead of flowing in a continuous stream through the engine, the intake air smacks into the closed throttle plate. The turbo, which is still spinning and producing boost because if it's rotational inertia keeps producing pressure, and the intake stream, caught in between a rock and a hard place, jumps in pressure. In fact, you get a high-pressure shockwave that travels from the throttle plate back to the compressor vanes, that once it gets there, is a little like poking a stick into the spokes of a bike wheel.

The repeated shock is hard on the compressor vanes and the shaft bearings, and in any case acts like a brake, slowing the turbo, and requiring it to be spooled up again.
The BOV sits in between the turbo and the throttle plate, and if it detects the shockwave created by a shift, vents it elsewhere - either to atmosphere, or back to the inlet side of the turbo.

So, we lost boost pressure, but we kept the turbo spooled... tough to say without a dyno if that was a fair trade on a race vehicle. On a street vehicle, it was definitely a good idea, because we spared our expensive turbo a mechanical shock.