In certain fights tank lb is a necessary strategy to survive the bosses ultimate attack, like the above post mentioned T13 and A12s. There are certain fights where it may not be absolutely necessary but it is highly recommended in order to ease a certain mechanic like ultima hard and A8s. That being said, a quick reaction by a tank using lb can save a run in different encounters as well, for example, going into an Akh Morn in nidhogg ex with several players down could be a wipe because damage is shared between everyone in the stack. A clutch tank lb3 can save the run in that situation provided you don't have a whm alive that can handle that type of damage.