The Undaunted passive ability gives a boost to HP, Stam + Magika for each different weight of armour worn.
Sometimes it is necessary for the DDs to utilise this passive in order to reach a 'sensible' amount of HP for trial content - especially if they are using drinks instead of foods, which can result in HP being too low.

Adding a piece of Heavy to an otherwise light armour set, gives better survivability through higher armour rating, HP, Mag + Stam from the undaunted passive, plus any of the passives you choose to invest into in the Heavy armour skill-line.