All of the under covers and the wheel well covers help smooth the flow of air under the car and the primary engine undercover helps ensure that air coming in through the grill is pushed through the intercooler, radiator, oil cooler and a/c condensor instead of 60% just going out under the car (and increasing front end lift at higher speeds). Going without it out of laziness makes as much sense as going without the radiator shroud for the same reason.
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