As a full-service network carrier, Atlanta-based Delta Air Lines operates a traditional hub-and-spoke model, meaning that they operate thousands of daily flights to and from their major hubs, which are located across the country. If you are looking to fly between two destinations, neither of which is a 
Delta Air Lines hub, you will likely have to connect somewhere along the way at a Delta hub. For example, if you are looking to fly from Philadelphia International Airport (PHL) to Miami International Airport (MIA), your most efficient option with Delta will likely be to connect at Atlanta Hartsfield-Jackson International Airport (ATL).