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Usually the wastegate is activated when the pressure reaches a certain level. An electronic wastegate can be opened at all times to provide said bypassing effect at cruise to improve fuel economy. One might wonder, a turbo is supposed to recycle waste energy in the exhaust, so why does it even create backpressure? Well a turbo's main objective is to increase power, and then it needs to be driveable as well. For driveability (turbo lag/transient response) the turbine is extremely restrictive to be able to quickly produce boost.
Engines with continuously variable intake duration can benefit from the pressure produced by the turbo that would otherwise be wasted via blow off valve.
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