People possess Circadian rhythms, they operate on a 24-hour cycle, and about half that time is daylight. Going to the place where that (work) activity occurs follows a pattern: Leave home early enough to arrive at the destination at a desired time. Do something there. Leave there (after, say, 8 hours) and return home. There are many complexities.
The graph has two peaks: morning and eventing. These peaks are the “rush hours” of common complaint, when more people want to use the transportation system than capacity is immediately available, leading to congestion. This graph shows both the supply provided by the public transport system (more seats are made available during the peak) and the demand of users. The supply clearly responds to the demands. The afternoon or evening peak is usually higher (and almost always broader) than the morning peak, as we organize more activities after work than before.
This pattern ensures some set of people (peak commuters) are generally at work at the same time, which reduces inter-personal coordination costs. If we are generally in the same place, we don’t need to pre-arrange meetings, since we can easily visit each other’s office or run into each other in the hall. The price of this is congestion, since we do not (and perhaps cannot) provide enough capacity to satisfy the peaks of demand when we do not charge for its use. Most trips are not work trips, even during rush hour. However work trips, with their tight scheduling and longer distances, overload the system at peak times.