Shivak increases the amount of health raised and restored by Fortitude, with the size of the increase dependent on his level. Shivak is a Favor General available for purchase from the Oracle, if your level is at least 135.
Increasing max health could prolong the survival of ally attackers should they have low max health, but the single-use restriction limits its effectiveness. It is not recommended unless the cleric has all other class abilities first. Save your guild coins to purchase more useful survival-oriented powers such as Resurrect, Heal, or Dispel.
The benefit of Fortitude is that the cleric does not have to wait for ally to be damaged in order to apply the health buff. If cleric knows who are the active allies before the start of the battle, then cleric can immediately start applying Fortitude on those allies.