I don't what that option does. For those who don't know what I am talking about, it's that shield icon next to admiral portrait of the selected fleet in the main map. I have toggled it on and off and I have no idea what it has accomplished.
What does the "Guard" option do in Campaign?
I think this is just a way to mark a fleet when you don't want to move it
Each turn I check all my fleets, if they have movement points I know I have to give them order
If I want a fleet to wait to defend on invasion I enable the guard mode
This way when all my fleets have 0 action point, have the guard symbol or the battle symbol I know I can click on "end turn"
my money is on they wanted to have a notification for idle fleets like civs does for its units but then realized it would be more work.