It's not a matter of logic.

When there is one cashier and a dozen customers waiting, that tells me the store is too cheap or too mismanaged to have other cashiers scheduled.

The best thing a store can do for customer relations is to open additional registers when too many customers are waiting, it implies the customer's time is important.

I also enjoy discovering it was the store manager themselves running the extra register - it means they stay in touch with how things are done (and how hard the cashiers have to work).