If you have something expensive to protect, like a high-end theater system, then there is no reason not to buy one. Supposedly they keep the power right around 118-120v, never higher, so it's extra protection. Also, many have surge protection capabilities

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2006 BMW X5 4.8is Lemans Blue/Beige/Dark Maple
-Nav, Comfort Seats, Heated Steering Wheel,
Rear Airbags, AUX Input
2009 BMW M3 Space Grey/Black/Carbon
-all options