Arrivals/Departures

Just looking for some answers. I’ve tried tutorials and can’t find anything useful. What are the departure slots for? I think I understand that arrivals are for incoming planes, so in order to get more air traffic I need more arrival slots and more planes filling those slots. I’m confused on what departures are for, do I need planes in that category? It doesn’t seem to affects the planes departing so I’m confused on how it affects gameplay. Any info would be great!!

You can get your own planes in the game. And once you handled them completely… they will depart for somewhere and each plane occupies a departure slot.