Can someone explain to me the point of Cull?

I literally never ADC, so I didn't realize until recently exactly the wording of the Cull item, and its a little confusing. I always understood it to be an item you buy, kill a bunch of minions, then get burst of gold, but reading it for myself it doesn't make sense. The item costs 450g, and gives 1 extra gold per minion kill, then 350 bonus gold after 100cs soooo at the end of it you just get 450gold, the initial cost of the item. What am I missing, this item makes no sense.
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