It's the hub-and-spoke model. For n cities, there are n^2 possible connections. Most of these will see hardly any traffic, so a direct flight would only happen

It's the hub-and-spoke model. For n cities, there are n^2 possible connections. Most of these will see hardly any traffic, so a direct flight would only happen once a week or so. With hub-and-spoke, everyone coming from Small City A will fly on the same quite-frequent flight to Atlanta, regardless of their intended destination. In Atlanta they'll all transfer to their own flights, going back to the individual smaller cities.