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As Naked Whiz reminds us on his awesome website, it's actually PLATE Setter, not "Place", while it is merely semantics, using the word properly, more accurately describes what it does.
Now, having said that, by putting the plate in, think of it as cutting off the direct contact of the flame from the food. Basically, when you use the platesetter, you are converting your grill into a wood fired oven.