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Even if the overall mean time ends up being the same, you are reducing variance, making wait times more predictable.

There is a good amount of psychological research on waiting, and people in lines are much happier if certain things can be relied on. One of these things is fairness, as in, my wait time should be as long as everyone else's. Another thing is knowing how long the wait will be, which reduces anxiety over the wait. Single queue feeders enable those two things to happen much better than multiple queues. So even if the math ends up being the same either way, the customer will likely be happier with the single queue.