Why do car companies sell in different countries under different names?

Why are companies like Chevrolet selling their cars under names like Holden in Australia? I'm not sure if Chevrolet sells in Australia but I do think they do. If they already sell in Australia, what's the point of selling the same thing under Holden? Another example is some Ford cars are sold under Lincoln, and Chevrolet sells the same thing under GMC. Buick and Opel? What's with this?

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