Why do destroyers need to leave on bug missions?

I understand from the game side of it but what is the in game reason. You could say the automatons are returning fire but what about the bugs.

Is their a bug like in starship troopers that can fire into orbit? If so it would be nice to see on the ship. Also we should have missions that take out those bugs.

If their isn't a reason and they don't want to add new enemies, I would like to see the timer not count down to your ship leaving but too enemy reinforcements arriving. Have your normal spawn rates for the mission while the time is counting down, but once it hits zero the bugs start flooding in. Kind of like they are currently in a normal mission.

I just see no reason for the destroyer to leave.