Slot machines are usually common in casinos and it has to do with slotting coins into machine before it can work. The idea of a slot machine is seen in the parking lot machine, the payphone, it's all about slotting in coins for the usage of the machine.
I think the idea of a slot machine helps the economy in preserving coins in the economy. People who are interested in using the slot machine will have to keep coins for them to access to the slot machine.
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