New to game

can someone explain to me about unlocking the arrival and departure slots and tiers. Why do I do this? What happens when I do? When game suggests to unlock a new one, should I do it? Which is more important, unlocking an arrival or a departure slot? I cant find any info on this on the forum - if there is, please let me know where it is.

It has been added to the game. Basically, it is that to unlock destinations from which planes will arrive and you can send planes to that airport. At first, limited amount of gates and destinations will be unlocked. Since you can build more gates more planes can come but there are limited destinations from where planes can come, so you have to unlock them to get more planes which results more money.