It's two sides of the same coin. In your example management only adds more cashiers if the queues have become unacceptably long (they've tripped a soft limit). The queues are serving to accommodate irregular shopper purchasing patterns because shoppers don't arrive consistently throughout the day. So really they are serving to smooth over bursts. Adding more cashiers is simply a response to queues hitting an unacceptably long length. If management had infinite cashiers and you could add them instantly then you wouldn't need queues. But, and this is especially obvious if you consider the scenario where all registers are staffed, then you still need queues to handle bursts. Costco is a good place to experience this.