&gt; If instead of queues we had stacks (last in, first out), we would no doubt see people organize themselves into the stack in such ways that other time-wasting externalities would arise.<p>The purpose of a line is that you have an inconsistent flow rate. If you always had less than one person arriving per minute and it took less than one minute to process one person then there would be no need for ordering the people waiting because there would be no people waiting.<p>The trouble arises when that isn&#x27;t what happens. At 10AM there is no line at the lunch counter but by 11AM or noon there are more people looking for food than there is serving capacity, so a line forms and remains until 1PM or 2PM when the rate of new customers has fallen back below serving capacity for long enough to process the backlog.<p>A queue is inefficient because it causes everyone to wait e.g. 15 minutes during the busy period. If you use a stack instead of a queue then a person who arrived at 11AM and sees the number of people in line in front of them continue to increase will correctly deduce that continuing to wait is futile. They won&#x27;t be served until ~1PM when the entire backlog is filled, so assuming they aren&#x27;t allowed to reposition themselves in line <i>immediately</i>, they might as well just leave and come back later. And maybe resolve to get there by 10:45 next time, when demand is lower and everyone is still being served immediately. Which is where the efficiency comes from -- more people arrive either before or after the rush to avoid getting stuck at the bottom of the stack, so the wait time goes down.<p>The trouble with it is that it lowers the average wait time but it makes the wait time for any particular individual completely unpredictable. You could end up being second to the top of the stack for several hours while everyone else is served immediately.