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IAmTheStig

The 370z with the sport package is advertised as having "zero-lift" aerodynamics with a coefficient of drag of 0.29. These additional pieces are required to achieve this.

From the Nissan Newsroom press kit:
"The coefficient of drag remains 0.30 (0.29 with the Sport Package). The 370Z also provides zero front lift and zero rear lift when equipped with the rear spoiler, thanks to extensive aerodynamics efforts - including an "inverse area" front bumper design which smoothes the flow of air to the sides of the vehicle, while the rear design maximizes the control of air flow from the roof to the rear hatch.

Performance body pieces are available as part of the optional Sport Package, including a front chin spoiler and rear spoiler."

You don't need to use them, but you're not getting all of the benefits of the sport package without them.