Final boss fight of CM. the small adds target 1 person. Hate cannot be taken from them- so if their target is a healer, you "should" cover them if they are not being slept.

Final boss fight of bahamut a coil turn 1- some people use a strat where they let the snake recombine. Then the off tank will hallowed ground and cover the main tank.

If there is ever a time an off tank is in a boss fight and they can predict a big hit coming (duty finder Titan is a good example)

Cover is not an ability you will use frequently, but having it available can be clutch