You can control the movement of a more complex collection of elements with a simpler one. It's just a simple tool. It's like asking in what situation would you use extrude. There is bunch. I guess the short answer is anytime you want to move something/somethings in a non standard direction. You can create objects just for a reference. Sorry if I' m not being clear.Maybe I can come with a better explanation later.

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"The true sign of intelligence is not knowledge but imagination".

Albert Einstein