Walmart is also smart about this. They algorithmically determine how many cashiers to have at certain times of the day, under the assumption that even though some people may give up and leave instead of waiting in line, its still more profitable for that to occur than to staff more cashiers for everyone to complete their purchases in a timely manner.

In the real world, its about profits, not volume.

Good point, with Walmart they will generate revenue no matter what, but to cut cost it makes sense to use it.