A journey between two continents frequently routes through a city that is neither origin nor destination. Those connecting points are engineered rather than incidental, and several of them compete directly.
A hub converts routes into a network
Flying every city pair directly would require an impossible number of routes, most of them too thin to fill an aircraft.
Feeding passengers into one airport and redistributing them allows a carrier to serve many origins and many destinations with a manageable number of flights.
Each additional destination therefore multiplies the itineraries on offer rather than adding one, which is why hub networks grow in value faster than they grow in size.
Geography decides which cities can do it
A hub works when it sits close to the great circle line between the markets it joins, so that connecting adds little to total flying time.
Cities positioned between major population centres can therefore serve traffic in several directions, while a well-run airport in the wrong position cannot.
Aircraft range matters too, since a hub is only useful if both legs are within the reach of the fleet operating them.
Connection banks structure the timetable
Hub schedules cluster arrivals into a window followed by a wave of departures, so that any incoming flight connects to as many outgoing ones as possible.
Those banks explain why a hub airport is intensely busy for a period and comparatively quiet between waves.
They also explain layover lengths, since a connection is timed to the bank rather than to the individual passenger's convenience.
Connecting traffic is a national strategy
Passengers who merely change planes still generate landing fees, fuel sales, retail spending and employment without requiring the country to be a destination.
Several states have therefore invested in airport capacity and home carriers specifically to capture flows between other regions.
Competition between such hubs is direct, since the same passenger could be routed through any of them, and price and schedule decide which.
What it means for choosing an itinerary
A connection adds risk, and the relevant question is what happens if the first leg is late, which depends on whether both legs share one ticket.
Minimum connection times published for an airport reflect the physical distance between terminals and the checks required, and a tight legal connection is not necessarily a comfortable one.
Longer layovers at hubs with good facilities are often worth choosing deliberately, particularly where the alternative saves an hour but removes any margin for delay.