People who work in factories, offices, hospitals, schools and so on all need to be there at the same time. For example, how would a factory production line of cars work, if half the people who worked on it just turned up when they liked? No, they must all be there at the same time to make it work efficiently.

We normally have a term for these twice weekday movements of millions of people (billions if you think world-wide). We call it the ‘rush hour’. Of course, it is not really exactly an hour, and it is more concentrated in the morning and more spread out in the evening, but the term gives the idea.